TALLAHASSEE, Fla. (AP) - As children around Florida head back to school, the toughest requirements of the state's contested class size amendment will go into effect.

The start of the new school year this week means districts will be required to meet strict limits on the number of students in core classes. Pre-k through third grade classes can have no more than 18 students. There will be a 22 student limit for grades four through eight, and a 25 student limit in high school classes.

Previously, class size averages were calculated at the schoolwide level.

The Florida School Boards Association plans to file a lawsuit challenging penalties for not meeting the state's class size requirements. Voters will decide in November whether to keep limits at the schoolwide average.