As long as they are universally applied, schools and universities can make any policy rules that they choose, even if the rule seems to be unreasonable, unfair, or unpatriotic. Perhaps if you contact housing staff management, you may learn the purpose for the rule. You may also be able to get the housing staff to reconsider its request and allow you to display your flag. Before you approach the housing staff, you should review the rules provided for you when you moved into your dorm. If you do not have a copy of the rules, one should be made available for you.