Re: Limiting Class Size
If ALL class sizes are ALWAYS the same, you could use a literal value for class size. I don't recommend it but you could. If class size varies by class, then you really need to add a field to the class table so you can hold the limit for that particular class.
If classes are limited in size, then how can you just add more students? If you can add more students regardless of whether or not the class is full, what is the point of maintaining a limit in the first place? It might make more sense to let a human decide rather than the computer. In that case, your roster form can display the number of students currently enrolled and a real, physical maximum beyond which you cannot go. Then the human can decide whether to add the student or start a new class.
A picture of your schema would be more useful. I'm pretty sure that your tables are not correctly defined to do what you are asking to do. Open the Relationships window and arrange the diagram so that all tables are visible and all the fields are showing in every table. It is important that we see everything, if you can't make that happen, then uploading just empty tables will give us a starting place. If you want help moving stuff around and actually normalizing the schema, you WILL need to upload a database because no one will want to retype the whole thing.
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