Public Holiday Considerations for Bakri Id
Bakri Id may be a public holiday if it is declared by the state, but this typically applies to state government and banks if declared under the NI Act. For all others, it is their choice whether it will be a holiday or a working day.
In some states, there are Festival Holiday Acts which may apply, providing protection. In any case, most enterprises consider Bakri Id as an 'optional holiday' applicable only to the religious minority affected.
If the school declared it as a working day and you did not attend, they can deduct salary for that day. If they had earlier declared it as a holiday, then it is unfair, but the question is whether you can fight it and whether it is worth a fight.