The only way to ensure IDs are not duplicated across tablets in an offline manner is to generate a really long ID (e.g., use meta/instance_id which is 30 characters). But that's going to be painful to re-enter.
Grzegorz is on the right track that you want to include device ID, but it might be simpler to assign a letter to each tablet (e.g., A-D) and then add date and time.
How much of the date/time you include depends on when you are collecting the data. For example, if you survey all the students in the same day, then you really only need hours and minutes and seconds. But if you survey one a month, then you really only need the month to uniquely identify students.