Do you need to use JavaScript? Using a Nanoflow would be a more Mendix way of doing this.
If you do need to do this using JavaScript then take a look at the addReference method of MxObject. That should let you set the association of a Student using the guid value of the College.
https://apidocs.rnd.mendix.com/10/client/mendix_lib_MxObject.html#addReference
I hope this helps.