Lead with the post and your qualifications
Schools receive many applications for each vacancy and often sort them by eligibility first. Name the exact post and subject in your subject line and first sentence, and state your qualifications - degree, B.Ed, and CTET or TET where relevant - straight away.
Mention the boards you've taught (CBSE, ICSE, IB, Cambridge or a state board) and the classes, so the principal can see at a glance that you fit.