Hi there,
In iSAMS, a Form can be thought of as a subset of Year Group. For example, imagine there are 100 students in Year Group 4- they would be divided into 4 Forms of 25 students each: 4a, 4b, 4c, 4d.
Additionally, some students (generally Primary) are timetabled Form-based teaching. This means that the kids in 4a will go through all the classes in their day with the same kids. English, Math, Science... all with the same 4a kids.
Set-based teaching is when (generally Secondary) kids have a homeroom, i.e. 10a, but thereafter will go to Set '10 Bio AP', in which they might be mixed with any number of other kids from 10a, 10b, 10c, 10d.
Then they move on to Set '10 Music' which may have an entirely different mix of Year 10 kids, etc...
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As to the 500 error, omg yes. If there is so much as a typo in your import file that doesn't match with what's in iSAMS it just explodes rather than telling you that there is a data mismatch. It was a massive headache for me to figure that out.
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Check out the iSAMS Discussion forum for help. It's not very active but it's something:
https://discussion.isams.com/
There are also some helpful webinars. You might refer to the rollover specific ones here:
https://icommunity.isams.com/media_center/folders/4313b21c-cbb1-44f4-980d-45c1f3d31419
Good luck!