I am 17 and lived for 9 years in London; I have lost my American-English accent.
Now I have returned to my home country, and I just can't get through to them "Stop making fun of the way I talk!" ( Here in my country, AmE is common )
I was mocked by other students in my class and the conference hall. I don’t want the others to laugh at me.
I’d like to say a few words in order to make them feel a little ashamed!
I was mocked for having an English accent when I first moved to Canada. I was younger than you are, and it was really upsetting. I can't control how I speak! In my case I lost the accent very quickly as a form of self preservation.
I don't recommend mocking them back. If nothing else, it shows you thinking mocking how people talk is a fine thing to do. What's the point of that?
You have two real choices: rise above it, or open up about it. A third not very realistic choice is to make different friends. I mention this in passing, because there are plenty of people who wouldn't be mean to you, you are not stuck with this group of friends.
To rise above it, you just smile. You acknowledge that you don't sound like them. You say things like "living abroad has changed me in lots of ways, mostly all good. The accent is just one part of it." You say "I hear girls love British accents" (or "boys", as appropriate.) You say "Americans think everyone with a British accent is positively brilliant!" In every way you consider your accent a good thing: a marker of your broader world, wider experience, different education. (To be honest, this is the root of the teasing - your friends know this is a lovely thing about you and want to say it isn't so that they feel better. It's like teasing someone for being tall or getting the best marks.) Don't be a snob. Don't put them down. Just enjoy this thing about yourself.
If that feels too brave, and these really are your friends, then open up with them. Don't laugh or smile. Look right at them and say "hey, I can't help how I talk now. My parents took me there and this happened. It's not under my control. Don't bug me about it please."
Your plan to make them feel ashamed is not a good one. You want them to either feel empathetic, and not mock you at all (what with being your friend and all) or to realize that you don't mind what they say about your accent, because you like it. Those are achievable goals.
Make a joke, so instead of them laughing at you, they're laughing with you. You can say something satirical to make light of the situation, such as:
Contrary to what you might believe, people in the UK actually speak pretty good English.
As of course you (and hopefully they) know, England is where English comes from, so the British naturally assume theirs is the proper English accent, and occasionally make fun of us Americans for speaking something (to paraphrase Douglas Adams) "almost, but not quite, entirely unlike English".
In a light-hearted, fun way, turn the joke back on them. Examples:
Pretend not to understand their "horrible" accents (with their weird "a" sounds and strange terminal "r" sounds, and bizarre, made-up words like "sweater"). Suggest they invest in remedial language lessons, and give them a short lesson in how to pronounce, for example, "aluminum" or "vitamins" or "schedule".
When you talk to them, exaggerate their American accent to the point of absurdity. Here's a skit from Monty Python's film "The Meaning of Life" (with varying degrees of success) as an example.
Every time someone mocks your British accent, bring up some random (and possibly made-up) fact about the UK:
Did you know the United Kingdom recently named chicken tikka masala as its national dish?
The crumpet, which the British eat every morning for breakfast, was originally designed as a weighted throwing disk to protect themselves from early-morning marauding Vikings.
and so on. Again, use humor to defuse the situation. Be clever, and eventually you'll all be laughing together, instead of you being the odd one out.
