The letter should not exist in the first place..

If it were to be written,it should touch on these few points

Pongal is one of the celebration by Indian ppl
Promote muhibbah spirit, so everyone is okay to join
But for Muslim students, they are encourage to join but should adhere to the rules as stated in the original photo when joining.

All in all, the writer don't really know how to express the idea. That's why it's perceived wrongly.