I just taught a Helitrox class. I'm assuming it will be my last one. The amount of He in the mix is close to being inconsequential (END of 113 on a 150' dive, at best), but more importantly getting He for open circuit is basically a non-starter at this point. Even if I can get it for class, the students won't be able to get it to conduct dives. They'll be stuck doing 150' dives on air, which they wouldn't have done before.
Just go CCR. It will change your diving point of reference so much, that mix before or after doesn't really matter. Getting on the loop sooner than later is more important than if you have a mouse fart of He in your mix or not. I just did a CCR course for a Prism, cross over for me, first CCR for the other three students. It was an air dil class, even though all three of them are mix certified on OC, and I'm normoxic CCR.