It's a pretty common combination, although I've never heard it called a "gradeschool speedball"...
It's actually safe to do as long as you keep your dosages sane (don't go higher than you would normally for each drug - in fact, if you're planning on trying this, lower the dosages of each drug to gauge your effects.
Speedballs (of all kinds - I'm talking opiates + stimulants) are quite euphoric but its easy to get carried away with dosages.