In many cultures and schools, especially in younger grades, addressing a teacher as "teacher" is perfectly acceptable and even expected. It's a respectful and common form of address.
However, in some schools or higher grades, teachers might prefer to be addressed by their last name (e.g., "Mr. Smith," "Ms. Jones," "Dr. Lee") or even by their first name. The best way to know is to observe how other students address the teacher and/or ask the teacher directly how they prefer to be addressed. This shows respect and attentiveness to their wishes.