Basic Bio won't get you anywhere near the level of knowledge you need to grow anything worse then the flue.
Basic chemical knowledge, conventional explosives, and large amount of faithfull willing to die and go to heaven are a lot more effective, even as a weapon of terror. Hell, attach a big bag of flour to the bomb if you want to scare people once in a while. Use a thermite thingymagic if you want spectacular imagery and not much casualties. Hell, create HCN (hydrogen cyanide) if you want lethal gasses. All are easier to create in a school lab then a biological weapon. I can whip up a manual for thermite in a few minutes, and can probably think off some other things to go BANG instead of 'psssh' and burn through the floor as well. Biological stuff is just a lot more sensitive. And harder to controll.
Plus, the real problem is the dispersion. That sect in Tokyo had pretty good Anthrax, but couldn't get it delivered. If something is lethal enough to kill someone after 2mg or something, you'd need to inject him with those 2 mg. If you only have skin contact or breathing, you'll probably need ten to a thousand times that dose.
Media scares are more common then real threats, especially with nuclear and biological stuff.
@Kara: Chemical is a lot easier to dispearse, but a nuke is the most efficient method of killing of a lot of people quickly. Probably good ole TNT if you go for kills per dollar, actually. Seriously, if you look at the financial point, it's better to blow up a city then infect it with some disease. And as said above, dispersion is a ***** when trying to kill of a lot of people with biochem stuff.