it depends the context or your relationship with your teacher, if the teacher is your friend and you asking for go to a party with him, is no make sense using "usted", but if you are in the classroom with another students is better to use "usted" to asking o say something. in some countries in latinamerica is rude to use "tu" when you dont know to the person, the person is older than you or in the work the person has a higher position.

Yo creo que si el profesor es de algún país americano deberías usar la forma usted, porque allí es la norma habitual. Sonaría raro tratar de tú al profesor sin haber pedido permiso antes y podría ser considerado una falta de educación. En el caso de que el profesor sea español puedes empezar directamente usando el tú. En España la forma tú tiende a predominar sobre la forma usted en la mayoría de las situaciones. La verdad es que no es fácil saber cuando usar una u otra forma. Saludos

¡Muchas gracias por las respuestas!

Si yo fuera tú, empezaría usando "usted". Luego, si la relación con la persona llega a ser mas amistosa, puedes preguntar "Puedo tutearlo?" for men, or "Puedo tutearla?" for women, and if they don't mind, you can use "tú" from that point on. You can even ask them almost immediately, if they're more or less your age, because we tend to use "tú" automatically if we see that the person we're speaking to is our age. The thing with the "tú" and the "usted" in Spanish, is that there aren't too many hard rules, you usually use "usted" for officials, bosses, and people you don't know that you ask something on the street, or people that work in a bank or a government office, for example, even if they're your age or younger, we tend to use "usted". If you're a kid and you talk to a grown person, if you are grown, but you talk to an older person than you, people tend to use "usted", but, as you would realize quickly if you live in a Spanish speaking country, there are tons of exceptions, there are kids who say "tú" when referring to their parents, uncles, aunts, and even their grandparents, even though they usually say "usted" for most other grown ups, also, some people, even if they're talking to someone older than them, say a 35 year old talking to a 60 year old, you'd expect to hear "usted" but sometimes the conversation naturally flows towards using "tú". But again, I think the best policy here in italki, is just ask.

Usted es honorífico así que puedes usarlo con un maestro o alguien de mayor edad :)