How to address Head Teacher in an email?

[GrandmaSharksDentures]

I need to write an email to the head of my child's primary school. It's about a matter relating to fundraising / PTA.
Should I address her by her first name or by Mrs xxxx?

I'd put Dear Mrs XXX

Mrs Xxxx

Mrs or MS depending on her preference (if known)

Thanks. I emailed using her formal title.

Always formal.

A really good way to get right up their nose is to use their first name when they’re still using your surname.