I've seen a lot of your vids on youtube !
I used a EOS 400D with a Sigma 70-300 APO DG of my friend i don't have a dslr yet
The room have very wide open windows so it help when is day time but in night where most of lessons takes place its very difficult with the 400D to shoot specialy with the ISO noise at 1600 and the focus is very slow.
70mm (x1.6) is also a problem its too close i always have my back stick to the walls and push hoping i will have more space
, the 24-70 f/2.8 would be great i think or the 24-105.
And a flash maybe .