UKCasual

It is fairly mild and not a big deal, but most people probably wouldn't want their 5 year old daughter saying it to their great aunt Mary.

It is not an appropriate word to use when talking to a teacher, no. A "pretty mild" swearword is still a swearword.

Try saying "merde" next time in thick French accent.

"You're the English teacher, what word or words should I consider using instead?"

It's a less vulgar version of shit. You wouldn't use it in an email, but when talking it generally isn't a big deal.

you should avoid swearing at all in formal settings

I raised my children not on the word, but the intention of the word. When they were saying "crap you" they meant "fuck you", and once they were made aware of it they stopped (saying it in front of me, anyway lol). So be aware that people will pick up on the intention and not necessarily the word.

In the US (not UK, sorry!) It is entirely dependent on the family that raised you. In some households it's just another word, and in others it is a curse word. In my experience the households that consider it a curse are less common.

This caught me off guard when I was roughly your age and got in trouble for it when visiting a friend. I thought it was a regular word and was scolded in front of others for it. I had no clue what was going on because nobody would tell me what word it was for a while.