Does it affect the legal liability of the host of a graduation ceremony if a girl undresses during it, or if another girl admires her clothing and imitates her?

If you organize a party, you must prevent any reprehensible acts (munkar) that might accompany it, because whoever has authority over a place is obligated to denounce and prevent munkar with his hand. If you are capable of doing so, then there is no objection to holding the party. Otherwise, there is a concern that you may incur the sin of failing to denounce munkar.