Having opened this thread for the first time and reading from the original post, by post three I was going to suggest the Open via the Berlin move order...! TalJechin you beat me to it !

My first reaction, "Ask Markovich," turned out to be not very helpful.
I like the Open Spanish for scholastic players 1200+.  Black gets to play d7-d5 which should count for a lot in the Open Games.  Tarrasch, the Great Teacher, was a believer - that is enough for me.

Not that you'd face the Spanish much at a scholastic level, but maybe the Steinitz would be a good choice at this level. Granted it doesn't lead to wide open tactical positions, but it is easy to understand the basic ideas using the games of the old masters, and when White plays d4 the position does become semi-open with ...exd4 (I'd recommend this approach over the closed lines).

The Bird tends to lead to open, tactical positions but I wouldn't recommend it because it doesn't teach basic opening principles like fast development and keeping the king safe (in some lines you have to play ...Kf8).

I don't think the Open is a bad choice though - it's not like White players will know the details either. Black's ideas are fairly simple really - develop quickly, attack the e5-pawn with pieces and/or ...f6, keep your dark squares protected (...d4 is great if you can achieve it!), etc - and 6.Re1 (which I'd imagine would be a common move in scholastic play) is not hard to counter with a little bit of knowledge. Maybe the main argument for avoiding the Open is that it's hard to get an open position after 5.d3 or 5.Qe2 but kids playing this as White won't know what to do when the position opens up with ...d5/d4.