I keep getting alerts stating startup disk is full when it's not
Do they ever get the error? Find out who's account is actually the one causing it. Then log in as them and have them use the computer until it happens again. Once the drive is full, double click on the "Macintosh HD", switch it to "List" view, and go to View > Show View Options > Calculate all sizes. That will let you track down the mystery 63 GB and see exactly what is using it.