Me (14y male from Poland) got my english teacher annoyed/disappointed after saying the word "crap". I'm unsure if this is the right place to ask this question but is saying that word considered bad? I always thought it was pretty mild and not a big deal.
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.
I don't know of anyone that doesn't think of it as a very mild "bad word" in the US, and I've lived across most of the country. Growing up in a very permissive household can mask that, but if you look it up in the dictionary you'll find it labeled "mildly vulgar" or something similar.