高校三年生ですって英語でなんて言うの？

I'm in twelfth grade. I am a senior this year. This is my last year of High school!!

"I'm in my senior year of highschool." "I am a senior at my highschool." Both of these expressions are appropriate for what you are trying to say. In the U.S. when you say that you are a senior at something it tells me people that you are at the top of something. For example in you say "I am a senior at my highschool." you are telling people that you are in the last greade of highschool, this could also with University/College and at work. "Im the senior software engineer" meaning that you have been working the longest with that company.

There are a few ways you can say this in American English. For example, you can say "only one more year" which means you have one more full year to complete. You can also say "second to last". This again means that you have one more full year to complete. In American culture, you could also say you are a junior in high school. American high schools are four years in length. First years are called freshman, second year are sophomores, third years are juniors and fourth years are seniors. This would be a casual way to describe a high school student.

The Senior year in high school monumental. A stage in life of ending and beginning. Wanting to express that is expected. In America, we have grades starting from Pre-k (the grade before Kindergarten) to 12th grade which is the last year of secondary school. We simply say, “I’m in the 12th grade.” or “I’m a Senior this year.” Either way is fine and will work, as well as be understood.