You're allowed to celebrate religious festivals in school
All schools, whether state-funded or independent, are expected to provide for the spiritual, moral, social and cultural (SMSC) development of their pupils.
This includes learning about a range of faiths, traditions and cultures – part of which is learning about religious festivals.
As well as teaching pupils about these, you might also choose to celebrate them in school (for example, Christmas, Diwali, Eid). You won't be discriminating against pupils of other faiths by doing so (as explained in the DfE's equality guidance, paragraph 2.14).
What you celebrate will depend on the type and character of your school
It's up to you – though if you have a designated religious character,