I'm just looking through the School of Rock electric guitar parts. What do the markings UBER and PLEXI mean? I'm guessing they are amp settings, meant to sound like specific amps...
They are almost certainly to both distinguish between the two guitar parts and also indicate the two different guitar sounds.
"Plexi" refers to the classic Marshall model 1959 100 watt head. This amp was made famous by Jimi Hendrix, among many others and is one of the most popular amps of all time. It usually has to be turned up very loud to get distortion from it. When run loud in its "sweet spot", it has a crunchy overdrive sound, not a thick high gain sound. This is a classic choice to use for a rhythm guitar part that can be kicked into a lead sound with a fuzz pedal in front of it, the classic choice being a Dallas Arbiter Fuzz Face or clone or reissue of the same.
"Uber" refers to the Bogner Uberschall 100 Watt amp head. Unlike the Plexi, this amp has two channels, one that is usually run "clean" (no distortion) and the other designed to provide thick high gain distortion. It has a more modern sound.
I would interpret those two parts as primarily indicating rhythm and lead parts, with the Plexi taking most of the rhythm work with occasional leads and the Uber taking the lead usually and playing an occasional rhythm role. Also the Plexi would come out more for classic tracks like AC/DC while the Uber would stand out front for more modern songs.
Personally I would listen closely to Broadway cast recordings and use them to inform the sounds used in the original staging and decide where I want to recreate that versus tweak it slightly for the production in question.
After listening to some of the Broadway cast recording to see if I could clarify further, I would suggest not trying to get your hands on either a Plexi or Uberschall if you're playing guitar for a production of School of Rock. Seems like you need two guitar players who can both play electric and acoustic. For the electrics, I would want something lighter weight and more affordable that can do a good mid to high gain sound like a Marshall DSL head or combo. Two of those would be fine. If you wanted to have a little more variation in sound between the two electrics, then perhaps a Vox AC15 for the second amp would give you some gain without sounding identical to the Marshall. Having a fuzz or overdrive pedal or three to go in front of each amp will give you more tonal options for solos. I also did hear a Whammy pedal or harmonizer on one track.
Oh, and then I would decide who will play the "Plexi" parts and who will play the "Uber" parts and label each amp appropriately with some board tape or something, so that anyone with access to the score (e.g., sound engineers, orchestra leader) can just look and see who is who.