A DP subject choice is not just six subjects; it is a combination that must satisfy IB requirements and your school's own constraints. When combinations are checked after the deadline, every error becomes a follow-up conversation, and some are only discovered when teaching starts. This article lists the rules a coordinator typically needs to enforce, how to express school-specific prerequisites and exclusions, and how to catch errors at the moment a student submits.
Two layers of rules
It helps to separate the rules into two layers.
IB programme rules apply to every school running the Diploma Programme. Students take six subjects: one from each of groups 1 to 5, plus an arts subject from group 6 or, instead, an additional subject from another group as permitted by IB rules. At least three and normally no more than four subjects are taken at HL, the rest at SL. For anything more detailed, such as which subjects may be combined in specific cases, check the current IB rules and subject guides.
School rules come from your own offer and constraints: which levels you teach, how many HL subjects you require, which subjects cannot be timetabled together, which subjects need another choice. These vary from school to school and from year to year.
Your validation must cover both.
The rules to enforce
HL and SL count
Decide the exact count your school requires, for example 3 HL + 3 SL, and enforce it exactly. If you allow 4 HL for some students, decide whether that is a general option or an exception approved individually.
One subject per slot
Each student should have one subject in each group slot. If a subject can appear in more than one slot (for example, a science taken in place of an arts subject), it must still not be chosen twice.
No duplicates
The same subject should not appear twice in one student's choice, even at different levels and in different slots.
Level availability
If your school teaches a subject only at SL, a student must not be able to choose it at HL, and vice versa. This seems obvious, but forms often list all levels for every subject.
Prerequisites ("requires")
Some choices require another choice. Examples of the kind of rule schools set:
- a particular HL subject requires a specific mathematics course;
- a subject in one slot requires a related subject in another slot.
These are school decisions, often based on teaching experience or university advice. Write each one down with its reason.
Exclusions ("excludes")
Some subjects cannot be combined: because they are timetabled at the same time, because their content overlaps too much, or because of IB restrictions. Examples:
- two subjects that the school always places in the same block;
- two courses that your school does not allow together.
Quick reference
| Rule | Source | Example of an error |
|---|---|---|
| Exact HL/SL count | IB and school | Two HL and four SL |
| One subject per slot | IB and school | Group 4 slot empty |
| No duplicates | School | Same subject in two slots |
| Level availability | School | HL chosen for an SL-only subject |
| Prerequisite | School | Subject X without required subject Y |
| Exclusion | School | Subject X combined with subject Y |
Why check at submission
There are three moments to catch an invalid combination: when the student submits, when the coordinator reviews after the deadline, or when timetabling or teaching starts. The cost rises sharply with each step.
At submission:
- the student is already thinking about their choice;
- they can fix the error immediately, with a clear message explaining why;
- no one else has to chase them.
After the deadline, the coordinator has to identify the error, contact the student, wait for a reply and re-enter the data. In September, an invalid combination may mean a student has to change a subject they have already started.
Writing good error messages
Validation only helps if the student understands what to change. Compare:
- "Invalid selection." Not helpful.
- "You have chosen 2 HL subjects. Choose exactly 3 HL and 3 SL." Clear.
- "Subject X cannot be combined with subject Y. Change one of them." Clear and actionable.
Each message should name the rule, the subjects involved and the fix. Server-side validation matters too: checks that run only in the browser can be bypassed, so the final check must happen where the data is stored.
Keeping rules up to date
Rules change. A new teacher allows a subject at HL, the timetable changes, a subject is dropped. Before each cycle:
- Review every subject's level availability.
- Review prerequisites and exclusions with the timetabler and heads of department.
- Remove rules that no longer apply; outdated exclusions frustrate students for no reason.
- Test the rules with a few sample combinations, including edge cases.
Validation and corrections
Validation is not only for the first submission. In a corrections round, every change must be checked again: moving from a non-viable subject level can break the HL count or an exclusion. If the corrections round is restricted to subject levels that reach their minimum, that restriction is a rule too. See minimum group sizes for DP subjects.
Valid combinations are also the starting point for building option blocks; a grid built on invalid data will not survive contact with real students.
Checklist
- IB programme rules and school rules listed separately.
- Exact HL/SL count decided, with any exceptions defined.
- Level availability set for every subject.
- Prerequisites written down with reasons.
- Exclusions agreed with the timetabler.
- Duplicates and empty slots checked.
- Validation runs on the server at submission.
- Error messages name the rule, the subjects and the fix.
- Rules reviewed and tested before each cycle.
- Corrections validated with the same rules.
Frequently asked questions
Should the school allow 4 HL subjects?
That is a school decision within IB rules. Many schools require exactly three and treat a fourth HL as an exception approved individually, because of the extra teaching time and workload.
Where do exclusions usually come from?
Mostly from the timetable: subjects that will run in the same block cannot be taken together. Some come from content overlap or from IB restrictions; check the current IB rules for those.
Can a subject count in two different groups?
A subject can be offered in more than one slot, for example a science taken in place of an arts subject, as permitted by IB rules. It must still be chosen only once per student.
What if a student needs an exception to a rule?
Handle it as a documented, individual decision by the coordinator, rather than by loosening the rule for everyone.
See MyIB with your own subjects
MyIB is independent IB subject selection software for schools running the IB Diploma Programme. The school sets its catalogue, level availability, exact HL/SL count and "requires" and "excludes" rules per slot. On submission the server checks the HL/SL count, duplicates, level availability and school rules, and shows clear error messages. Book a demo to try it with your own rules, or see pricing. For the full timeline, read how to run DP subject choices.