I know this topic has been discussed a little on the JV thread as well. Your son is most likely playing JV as most varsity players would be too old to play a spring club season as well. I know RCSD#1 has a rule that only Freshmen and above are allowed to play Varsity soccer. Most incoming freshman would be playing in the U15 age bracket and thus would not have a spring club season.
As to your point first and foremost club comes first. Most coaches allowed my son to miss a club practice only for a JV game. This does cause issues because a coach could find his practices dwindle due to a large number of players having games. We never missed a club game for any JV activity. The only real difficulty came on weekends where possibly a JV tournament was scheduled as well as club games. We were able to work through the situation with the coaches but remember coaches are very reluctant to hear excuses about attendance. I could see some coaches affecting playing time accordingly. For the good player that really isn't an issue but the marginal starter may suffer under that scenario. This does put a strain on the player in that 2-3 times a week they will have two practices. The rewards far outweigh the risk concerning involvement with both club and High School!
