Is expiation obligatory for one who falsely swore under pressure from the teacher not to maintain prayer, and then did not maintain it?

If you uttered an oath and then broke it, expiation becomes obligatory upon you. You must maintain your prayers, as neglecting them is among the things that lead to destruction. Do not be overly strict with yourself regarding matters of purity and prayer due to obsessive whispers; for that is one of the devil's ways to turn a person away from obeying Allah. Seek assistance from Allah Almighty, be patient, and frequently remember Allah Almighty.