I don't actually play rugby, I only got one season in when I was about 20 y.o. before a knee injury ended my very short rugby career! However, training for rugby is something I have thought a lot about, and the principles I mentioned are things I have learnt here at T-Nation which you can apply to many sports needing the same athletic qualities as rugby.
My recommendation for a "one-off" bulking phase does not mean you stop doing weights to increase muscle bulk when this phase ends, what I meant was the period from just after the season finishes to about 2-3 months before the season starts is the best time to really concentrate on bulking as the main focus of your training. This assumes the season finishes about September. That way you can focus on bulking and not worry about some fat gain because you will have time to get conditioned again pre-season. If you have a longer pre-season then by all means do multiple bulking phases broken up by something else e.g max strength.
Also, even though you are "just" playing at high school level, use the time now to develop good training habits, the fitter you get now the easier the transition is going to be to a higher class of rugby when you get out of high school. The game doesn't change, only the skills and physical nature changes as players get bigger and stronger, skills take time to develop, so try and eliminate the weakness you can work on now (size and strength) while you have the chance.
When you move into more sport specific training in the few months before the season starts then you do have to adjust your weight training. The programs suggested such as WS4SB and Athlete's Pendulum Training are two that allow you to develop the athletic strength qualities but not exhaust you so much that you can't do the conditioning and SSP.
During the season it is best to back off a little bit, you are not going to be able to add muscle due to the demands of training and the weekly games, plus you won't be able to rest as much as you need to add muscle. It has been recommended by some of the coaches on T-Nation to have one day early in the week of heavy training in the limit strength and functional strenght ranges, and then another day closer to the game of lighter, more explosive exercises e.g cleans, power snatches.
There is a good article on Dan John's website on planning the off-season training for rugby.
Go to http://danjohn.org/. Go to the archives of the "Get Up!" newsletter. Look up Volume 1 Issue 7. It should answer a lot of your questions and you should be able to see where a lot of what I have mentioned above has come from.