Assalaamu alaikum my dear,
What you have been asked to do is not respectful of either faith tradition. It implies that both holidays have no meaning, and are just an excuse for a party. To give an analogy, both secular holidays of Remembrance day and Halloween are close together in time. However, it would be unthinkable to celebrate both on the same day. These two days are very different in meaning.
I would suggest that you consult open-hearted and knowledgeable Muslim adults in your community, and that you find similar allies in the Hindu community. If both communities unite to respectfully and firmly point out this inappropriateness, I think you could make your point with the school authorities: Both faith traditions should be respected, and the celebrations held on different days, to honour their different meanings.
May Allah, the Guide, Help you find a just and harmonious solution.