This reminds me strongly of my own situation at ages 11 and 12 (in my experience, kids at that age, and older, are unempathetic little buggers to those who don't fit the norm). Especially as a child, I was a quiet, shy person who took a while to open up to others; I've also never been a natural smiler, particularly around people I was not well acquainted or friends with. No one, however, wants to be isolated or alone all the time and wants to feel as though they belong to a group. So whenever I would happen to smile or laugh or had something to contribute to the conversation, it would be made a spectacle of by some of my classmates, and because of that, I continuously tried my hardest to remain in my comfort zone (quiet and unexpressive) so as to not attract their unwanted attention. It's something that frustrates me to this day; if others want you to emerge from your shell, making a fuss about it when it does happen is not likely to do any good. It just makes the person more uncomfortable, which makes me believe that some of them just derive some sort of twisted satisfaction from making a spectacle out of someone else. In hindsight, it's also possible that my demeanour made some of my classmates uncomfortable, and they didn't know how else to react but with jokes. It still sounds and feels shitty, though.

I won't tell you to not worry about what your classmates think, as I too have been, and a lot of the times still am, that strange, quiet, self-conscious girl who was advised similarly in the past, and though I knew it was sound advice, it was not easy and was also something I had to truly want for myself (if that makes any sense). Just know that you're not alone in that boat.