I'm not sure there is a misplaced modifier, but there still is a problem. Students are 3rd person, you is second person. It is more common to use you in this instance. According to the new rules, you will not be allowed access to computers only if you arrive before 9 p.m.
Marginally I don't think According to the new rules is a dangling modifier. Others may not agree. You can get rid of the beginning of the sentence if you are not sure.
Answer: The new rules just came out. Those rules say that you will not be allowed access to the computers if you arrive later than 9:00 p.m.
