Doesn't "since I've been here" have to be corrected to "Since I got(came) here"? Present perfect refers to the present, I guess.
48)Mr.Lewis is a dance teacher...Before he left, one of his students said to him, "I'm sure the next teacher won't be as good as you are" Mr.Lewis was glad to hear this, but he said to her, "Oh,no! I'm sure he'll be even better than me" The girl then said, "No, Five teachers have come and gone since I've been here, and each new one was worse than the last!"
No. "Since I've been here" is the same as "Since I have been attending this school/dance class ..." The girl still attends the school/class so it's fine. If she said "Since I came here ..." it is just possible that someone might think she might "Since I arrived in this room at the beginning of the class". That interpretation is unlikely but you will hear "Five teachers have come and gone since I came here" or "... since I started coming here" or "... since I've been at this school" or "... since I've been a student here" or several other possibilities.
I would say "There have been five different dance teachers since I've been at this school and you're the best".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.