To wonder if this is inappropriate

[justignorethecat]

For a teacher to tell secondary pupils she is a lesbian?

It's completely appropriate in the context described and I would complain.

This is really sad. It shows we still have a long way to go.

FASH84 yes, teachers might have mention their partners and possibly their kids in passing, not to make a statement.
In that context, and in the context of the OP, it's absolutely acceptable.

As I wrote, it's nothing to be ashamed of. Starting the year by telling your pupils that you are a lesbian would not be appropriate. It wouldn't be any more appropriate to say "my name is Mr.. and I am straight". It's all about the context.

It would be the same with religion or whatever personal detail you can think of.

who told you off? I certainly would complain about it as a matter of principle. maybe speak to your union.

I told some girls off for using gay in a derogatory fashion and said I was gay and didn’t appreciate my sexuality being used as an insult and was told off for it

Why were you told off for that? YADNBU!

Why would it be inappropriate?