Terminated a lady teacher from a private management school with only oral communication

Dear Sir My wife joined a private management school as a teacher in 2017. She has not provided any offer letter at all. She was getting the least salary on due course. Her salary after all these six years is less than what the new joiners receive. Last year she requested for a salary hike which is at least equal to new joiners. The Principal said she will consider her application next year-means this year. Yesterday the principal called her to her office room and asked to stop working by March 31st because there is division fall. She accepted it and asked for a termination letter. But the Principal said there will be no termination letter provided and the proof of termination will be the settlement. When I enquired, I could understand that, even having more than 2000 students and 130 teachers, they dont give offer letter and employment contract to any staff and the staff who resigns go immediately on the same date. Apart from that the school asks another teacher who has 23 years service in the same school and resigned 2 weeks back to sign on a stamp paper to get her final settlement. I could understand that they are making lot of interviews to appoint new teachers and the reason for termination is not division fall but because she asked for salary hike. I would request you to advice me if the activity of the school is as per the law, and if not, what do you suggest me to do further to get justice?