His new hobby and female friend - be cool?

But the OP’s husband and his colleagues are two adult members of the teaching staff, not school children. Adults aren’t left alone with individual children in private spaces because of potential risk to the child in a situation with considerable power differential. None of that is relevant to two adult colleagues using a free period to play music together. Even if people are characterising the OP’s DH as a deluded innocent liable to be pounced upon by his cougar fortysomething fellow-teacher while he’s trying to work out a tricky bit of Rachmaninov.

Definitely meet her.
They must sound so good together by now.

Invite her around to play and have nibbles for Chritmas cheer.
I don't think you will feel jealous but more so very proud of the musical talent your DH is developing.

Maybe you'd like to invite him out to a Christmas Carols in a beautiful cathedral. They are amazing with pipe organs and choirs.

This
The fact that he isn't a piano or violin player would worry me

Spoiler alert, he would not rather play music with his non-musician wife than with a competent musician colleague. The idea is nuts.

Also two musicians practising or jamming or doing improv or making music together are not going to want someone in the room who's just observing them. That's anathema to any creative person.

It's thoroughly cute.
I would enjoy that so much. Be sure to listen to them sometimes. Do they play for functions?

Book them to play for your home at Christmas.

I do think the conversation about him watching out for if the relationship could ever be heading down the inappropriate path is wise. You can never (and he can never) be sure of what his friend is feeling.
As teachers they would be well educated to never find themselves in a small science back room etc alone with a student. That is best practise so he will understand your concern. Do they welcome other musicians into their space?
I would hope so.

Yes you are, if my husband (or bf) was this jealous, they would be shown the door

I think I'd be blunt but not unkind about it, I'd say I feel a little jealous of the time he's spending with someone else and see how he responds.
Also, I can only speak for the music practice rooms in my secondary school, they are usually cupboards with windows on a main corridor- so it's possible that it's a much less romantic scenario than you're picturing haha

That’s not a bad idea actually. Tell him you’ve been delighted for his newfound interest and get him some lessons for Christmas 😂😂😂 He may ramp up the practice time though to support the lessons

There's nothing you can do about it really. If you trust him you trust him. If you don't, the relationship is probably not going to work.

Why don’t you ask him to show you what he’s learned? If he’s hopeless, you’ll know he hasn’t been spending the practice time practicing!

I’m only joking but OP I think it’s normal (and exciting at the beginning) to find someone with a common interest. You said she’s a professional? She has probably talked about it in a really engaging way and spurred your husband on to take it up again. She is probably helping him improve and likes having someone at work she can talk to about her interest. I wouldn’t worry about him going to her house with another colleague (her suggesting that actually makes it sound like she is deliberately avoiding anything inappropriate) but if it’s just him and her and you feel uncomfortable I think it’s fair enough to mention it.

This is how affairs start. I wouldn't be happy at all.

Is there any evidence of increased communication outside of school? Texting her? Making more of an effort on practise days etc?

Could you take the bull by the horn and suggest getting him violin/piano lessons for him? Or for you as well? Ie. Take ownership of this , and make it a hobby you can do together? (It may be telling if he bats this idea down, if he only wants to do it with teacher friend).

Also, can you go with him in the practice session? Invite yourself along, as a couple?

the the that would have got me is that he didn’t come home all excited that he had come across someone to practice music with, and at school!

also, the going round to her house with one other person there … really?

expect the best prepare for the worst, I’d make sure that she met me, so she could see we were a couple in case he’s been diminishing your relationship to her.

from someone 100% trusting whose EXH is now married to a good friend and colleague!

It turns out they have met once a week for a month between classes (both teachers) to play together.

Were you aware of this or was it happenning without your knowledge?

I don't think you are being ridiculous at all. Emotional affairs happen, and sometimes they are more obvious to the ones on the outside.

🤣🤣🤣

Yes, I’m 52, and clearly my sexual life is a dim and distant memory. I have long since hung up a ‘CLOSED DOWN IN PERPETUITY’ sign with an arrow pointing downwards on my belt. 🙄😀

if she’s 40s wouldn’t she already have a family or be past that whole thing?

I don’t think OP has anything to worry about, but the ridiculousness of this sentence made me CACKLE! 🤣

I’ve had three separate men at my house playing duets, one man at a time, no safety in numbers. Never occurred to me that there might be 3 jealous women sat back at home.

Mind, one was a vicar.

if he was quite good before then it’s often just the pleasure of playing with someone else.

if she’s 40s wouldn’t she already have a family or be past that whole thing? Not to say it couldn’t happen but it’s not as likely?

I can see your worry though. Strange things can happen.

does he love music? What grade did he get to?

I guess it’s adult activity in an otherwise pretty mundane day. It’s probably quite exciting because it’s for his enjoyment not anyone else’s.

It turns out they have met once a week for a month between classes (both teachers) to play together

This is probably where you started to get a bit bothered - he should have told you he was meeting up with this woman, just the two of them at the time it began. Finding out after, makes it seem like he was hiding it, even if he was (most probably) doing nothing wrong.

Why are you only finding out now?