Fundamentals are for children, ages 4-7. They are placed by school grade because the physical and emotional development of the child is the main factor at work rather that "technical skills".
Tap students are accepted in 2nd grade. It is at this point that their coordination has developed far enough to where they can master the beginning level skills.
Jazz, because of the style of movement, and Modern Dance, because of the mental concepts involved, are not appropriate for students before 4th grade.
Pre-Ballet and Ballet begins in 2nd grade with class just once a week. Beyond the basic level, to become proficient at ballet, students need to develop very specific muscular and mental strengths. Taking frequent classes is necessary to acquire those skills. The greatest improvements are seen in those who take more clases. Ballet 2B and up students take at least 2 classes per week.