"Excuse me, are you Teacher Li?"
one thing is that if you address a person that is older, stranger teacher doctor by using 您 not 你 .
please provide as well traditional character set as simplified is not adequate
台 platform 台 typhoon
"请问" literally means "please - ask" but is actually a fixed word to start a question politely.
Besides, "Excuse me." can also be translated to "不好意思," which is a common phrase used as a request for an individual's attention, or as an apology.
Yes it is necessary. Without it, it would be "May I ask, you are teacher Li."
You could also say '是不是' which would help identify it as a question, and make 吗 optional in this case.