yes, they definitely can not deny you lunch. especially if its a public school. now they can deny you break between classes to go to the lunch room, maybe if there have been security issues. but they must let you take a break to get and eat your lunch, even if it's in your classroom.
Giving lunch breaks to students is a must according to the law governing schools. This is because students must take their lunch whether inside or outside the school compound whichever the school administration prefers.