Can a teacher leave children alone in classroom?

No, a teacher should generally not leave children alone in a classroom. The specifics depend heavily on the age of the children, the location, and local laws and school policies. However, leaving young children unsupervised is almost universally considered negligent and potentially dangerous. Even older children require supervision, especially in a classroom setting where accidents or incidents could occur. There are very limited exceptions, such as a quick, necessary trip to the restroom or office, and even then, appropriate measures should be taken to ensure the children's safety (like having another adult nearby or using a secure system).