Derived from the word "projects" which are the broken down slums of cities and towns. Crime, gangs, drugs and poverty run rampant in them.
"Science projects" refer to such neighborhoods with a strong concentration of meth labs and crack houses. Any drug that is cooked/refined instead of just being grown and smoked (weed).
It's a play off of the stereotypical science project we had to do in school. Both are playing with chemicals but one is a group of kids trying to get a decent grade while the other is a bunch of sleazy criminals aiming for some money or a fix.
"Did you see those five sketchy guys outside that broken down house??? I think they're dealers"