"You cannot see another student's test."
Obviously, you can't deny people their sight unless you blindfold them, but you can tell them that they are not allowed to look at something.
"Must not look at" would be clearer although I don't personally have any issue with cannot being used as a prohibition. That may be regional. Others may feel that musn't look or may not look would be more appropriate.
According to the Cambridge English Dictionary, cannot indicates either lack of ability -or- lack of permission/not allowed. It depends on the context.
can not/cannot = to be unable (できません) or not allowed to (Te form + は+いけません or いけない)
"can (PERMISSION) to be allowed to" (Te form + もいい)
"If you have the authority to grant permission, てもいいです or てはいけません would be appropriate."
p.s. The negative te form of the verb なくてはいけません or なくてはいけない means "you must do something"or "It's not okay if you don't do it."
"You cannot see another student's test." means that you literally can NOT see another student's test, as in all other tests are on the other side of a wall from you.
What they want here is "You MUST NOT LOOK at another student's test", although I'd rather it said "You must not look at your neighbor's test".
I agree that "you cannot see another student's test" speaks more to one's physical ability to view the test . But I accept "You cannot LOOK AT another student's test" as a statement prohibiting cheating, although you may not look at another student's test is a bit better. As a native English speaker (and former teacher :-)) I find "You must not see another's test" to be odd. What I see when I open my eyes is not under my control, but what I look at is.