Interesting. I was once on the board of a multilingual school (I live in a multilingual culture) and I wonder if your school is one of the ones we looked at as part of our research, JK日本語. I believe we visited one in Hong Kong and another in Thailand.
Learning more than one language has worked in our school, but there was a distinct and heavy concentration on one target language. School, however, is an immersion environment and age is extremely influential in the ease of learning a language. I would exercise caution in trying to extrapolate too much from that.
I have seen a countless number of foreigners (so now we are talking about adults) come through here trying to learn more than one language at once and the end result has always been that they speak "pigeon" in the end (fragments, set phrases only, bad grammar, ...). Not pretty.
So yes, I agree with all, I would encourage anyone to concentrate on one language until you are fluent (and can maintain that fluency!) before moving on to another.