Ouch.
OK, so there's no quick or easy fix for this, at least not if you have any fixtures that are not connected to dimmers. I'm guessing you aren't a specialist theatre technician either so I'll keep this simple.
Before you can start patching, you need to know what you have to patch. Unless your school is surprisingly state of the art I'd expect most of your fixtures are plugged into dimmers. These will just have a power lead - no DMX connection so you can probably spot them from ground level. If they are all like that then you can breathe a sigh of relief, 90% of the problem has gone away.
First off, find your dimmers. In a school, I'd expect to find a number of boxes looking something like this (which is the newest version, preceded by 3 previous generations). Of course it may not be Zero88 - other manufacturers, including Strand, made similar products (i.e. a box with a dozen sockets on the front with some sort of control/fuse/circuit breaker panel alongside). There will be a very thick power cable going into the box and a thinner control cable. You might find things like this instead - smaller boxes, less sockets but still with a mains cable and a control cable. Look at the plug on the thin control cable if it is a 3 or 5 pin XLR then your dimmers are (probably) DMX controlled - follow that control cable back, it should go to the desk, possibly via a section buried in the wall. It may go to another dimmer box, if it does then follow the control cable going into that until you get to the first one (the one where the cable goes somewhere else). If that first connection does go to the desks then smile and skip the next 2 paragraphs. You may have hardwired dimmers like these but, as it's a school I feel it's unlikely. If you have then what follows still applies (sort of) but you won't be able to see or follow the wires. If you are in this situation try and establish the make/model if your dimmers and come back.
If, however, the control cable is an XLR but it goes to a different kind of box (not dimmers) you need to identify what that box is. One with a network connection might be an ArtNet node. No network connection but two incoming XLRs and one outgoing is possibly a DMX merger. No network, one incoming XLR and two or more outlet sockets is probably a DMX splitter. All of these are good, they may give you headaches in the future but not with this issue. Skip the next paragraph.
If aren't skipping this paragraph then you probably have (very) old analogue dimmers, probably using non-XLR connectors. This sort of connection means there must be one or more DMX to analogue converters sitting between them and the desk. These will have an address, probably set with rotary or DIP switches. You can follow the next paragraph to establish what the addresses are but look at the converter boxes, not the dimmers. First, take a moment to contemplate the level of spending on such things in education and to check that the dimmers at least have reasonably recent PAT test stickers and make suitable arrangements if they haven't after all they have to be multiple decades old. Yo can also marvel at how good the engineering was that they have survived so long.
OK so now you have one or more boxes that have addresses, you just need to know what they are. Ideally you should have the manual and you may be able to get it from the manufacturer's website or somewhere like manualslib. If you can't find the manual then look carefully at the dimmer/converter box. There may be a display of some sort, even better it may display a number between 1 and 512 when powered up - good chance that this is the address. If not then you may have to start pressing buttons - go have another look for the manual, you don't want to change anything by mistake. Failing that there may be rotary switches and you can read the address from them. Finally, there may be DIP switches - you can read about them here but essentially the switches give you the address in binary.
Having got the addresses count how many dimmers you have on each address. A box with 12 sockets but only 6 breakers/fuses probably has 6 dimmers so patch 6 dimmers starting from the address on the box. Again the manual could be of help here.
If you don't have any fixtures that have DMX input then you are done. If you do then you will need to identify what you have (exact make and model) and try to find the manuals online. You will also need the addresses and the modes they are set to so get the Tallescope/Zarges/scaff-tower out and go climbing. Similar things apply to finding the address as it does with dimmers. If you get stuck come back to the forum with at least a list of make and model.
I see Keith at Z88 has made a shorter reply. If anything I've said contradicts him then assume he is right not me. No apologies for the length of this reply though - I have tried to cover most of the things you might encounter at a basic level and that takes a lot of words 😀