Probably possible to OD on anything if you really want to, but I've done both at insane doses, physically safe enough, you will get to the point where want it to stop long before you get into danger. erowid has ld-50s for lsd, no one would ever dream of approaching them. As to which to do first, whichever you can get first, although I prefer LSD, it is the standard by which all others are judged. Shrooms don't last near as long, reassuring if it gets to be a little much. Doing it in school was stupid, doing it there the first time? Reality check, you don't know what you're getting into.