This situation recurs often, and it presents a rather complicated Fourth Amendment issue. Firstly, there is the bright-line rule that a police officer may not enter a private residence unless she has probable cause to arrest combined with exigent circumstances. This explains why the officer ordered the occupants into the hallway. The question is whether there is a functional difference between an officer entering a residence and ordering the occupants out. I have successfully argued that there is not. Citizens should not be put in the position of having to disregard the orders of police officers in order to protect their Fourth Amendment rights. In other words, a college student should not have to decide whether he will disregard the order of a police officer or forfeit his Fourth Amendment protections.
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Whether the officer had enough information to barge into the room is a very fact-intensive inquiry (i.e., how was the officer's attention drawn to the room in the first place?). Exigent circumstances may have allowed the officer to barge in (i.e., a reasonable belief evidence of underage drinking would be destroyed is s/he did not barge in). This is, however, worth looking into further. Based on your post, you could have a valid motion to suppress the evidence.
You should consult local, experienced criminal defense attorneys directly. Criminal defense attorneys generally handle civil tickets, such as, underage drinking, as well.
Also, you should look at your contract with the dorm and any guidelines/handbooks related to the dorms and their use. These documents may have further information for you. (Although, the contracts/guidelines for dorms I have seen thus far all state the police need reason to enter your room and cannot just barge in. ) Even if these documents indicate you are allowing the police into your room anytime, you may still have a valid motion to suppress, as we cannot contact away our 4th Amendment rights.
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