Feel the vibe. When you first walk into that room of so many people, look for someone who gives off good feels, and isn't in a huge group of kids. A smile, a quick gaze in your direction, or someone absorbed in a good book.
Sit next to them. No need to rush into things. Give them a smile, a nod. Ask for a pencile sharpener. Let them know your there, but if they do or say somthing you don't like, you have time to go elsewhere.
Ask them a question. Whats the page number? What time is this class over? Have you had this teacher before? They may start a conversation with you. They may just answer the question. If they give a small smile or think about the question before answering. Thats a good sign. If they quick to push the answer and turn away. try again with someone new.