If I understand well, you don’t want to be able to select a student twice, > if a student is already in Test Class 1, you dont want to have it in Test Class 2, right ?
If Yes
Start to check if a people is already in a test class with this formula. If coda detect your students once in the test class list, the checkbox qill go to “true”
Then, custom filter your lookup (in option settings) in class table, you only want people that are not already in a class > witch checkbox “already in a class” not true
@Christiaan_Huizer meens in the option settings here, that will filter your lookup value. With his formula, you will only have available people. Different (more elegant) solution than mine, but same results !