Yes, as I mentioned in the video, this is possible, but if a student is advanced enough to do that, they can probably also just solve the problem. You can also see if they try to submit that as code fairly easily. If you require a library to be hidden, there are more advanced ways to obfuscate your code.
*** LPT: Make a copy of the Java file you are deleting, so that if you want to make changes later on, you still have the code!!! ****
but it is possible to uncompil jar and war to return java file. (with JD-GUI)
Yes, as I mentioned in the video, this is possible, but if a student is advanced enough to do that, they can probably also just solve the problem. You can also see if they try to submit that as code fairly easily. If you require a library to be hidden, there are more advanced ways to obfuscate your code.