Re: since I've been here
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 - correct capitalisation, punctuation and spacing make posts much easier to read.