I was working with a private school for 3 years starting from 2015 and was their permanent employee .My offer letter said that in case of leaving job either 3 months notice period or salary for that time.I left the job in 15th April 2017 because of better opportunities.They forced me to resign immediately as they were not willing to pay me salary for month of may (vacation time) which i wanted to give as part of notice period.Now the school is threatening me wth a legal notice saying notice period was not served so slary for 3 months needs to be paid.Kindly advise