A player moving from MS to Jv or Varsity usually occurs in a smaller district. It can be done to help fill out a need in a specific position etc. it can also be done to help give a player valuable experiences that they will not get playing Jr high teams. JV will play more games, usually have better coaching and more intensive training and competition. A players maturity and well being should be considered by the coaches and parents during this time to ensure it is what is best for all involved. Some towns have youth teams that average 10-12 kids per grade, when you lose kids along the way to music, other sports, social interests the need arises to add kids from another grade to support the program.