People can be absolutely crappy and cruel but the good news is that like you said, you have good friends who don't feel the need to mention your stutter nor put you down for it but just tough to find those pure-hearted ones who can see you more than just a person who stutters. I honestly don't blame you for walking out and not wanting to go back, that was absolutely callous, humiliating and unfair on you. I would've most likely had the same reaction. Everybody has something about themselves that they're insecure about but to ridicule somebody's insecurity of a speech impediment no less is disgusting and says more about their vile characters than your own. I can assure you that a lot of people are decent. I'll probably forever stutter saying my own food order but the older you get, the more you tend to realise that people can be patient with you and won't care, they'll still care about what you're trying to say. I really think you should be very honest with a teacher or a school counsellor of some sorts so your academic progress doesn't become stunted so you can be around much better people, don't let those toxic ones completely ruin your future!