In general public school classrooms are overcrowded.  Managing, much less teaching, 30 to 40 kids in a classroom is a daunting task. 

Most teachers welcome parents helping out with classroom tasks such as taking attendance, grading papers, making copies, providing individual care to certain students.  Volunteering to help out in a classroom also reinforces the importance you place on education with your children and helps give you a better appreciation of what it takes to run a school, manage a classroom, teach students and how very important teachers are.