More enlightened places are doing red seal in high school. Having two red seal trades, I am no longer sure this is a good idea. Perhaps in other places tradespeople are treated better than in BC. Mechanics in particular get the shitty end of the stick here.
Anyway, about obtaining a red seal, or working towards it while in high school. While learning about trades at a young age is certainly a good thing. To be stuck in one trade at an early age probably isn't. Especially when there are union jurisdictions involved. It gets complicated and expensive to change trades. Getting an overview of different trades, with some real work time in a variety of trades would be a good thing.
When I was in high school, we had extremely good shops, and got to play with a bit of everything. We even did sand casting in aluminum and brass. Many school districts don't even have shops in all their high schools, so kids don't get to try a few things to see what they might like.