You use the command promp and type
net send [computer name] [message]
You'll find that if the command prompt is disabled, on a school windows 98 network there is usually many ways to get it back.
I'll go with that train of thought and make a few suggestions.
Try holding down the windows key and pressing R, this should give you a run box, type command.com and press enter.
if that doesn't work...
try starting word and inserting an object, use the object packager to make a link to the command.com file that will be on the local computer, now use this link to run the command prompt.
use a copy of command.com on a floppy disk that you havebrought in from home.
if you can't get command .com to run, then forget about it, it's not srtictly needed anyway.
Try writing a batch file, in it write
net send [computername] [message] it'll be a pain that you hae to edit this file each time you want to send a message but it's better than nothing.
if you can't run the command.com, and you can't run batchfiles, then try creating a new shortcut to the file called net.exe and send the parameters [send [computername] [message]] with the options you can specify in the shortcut creation poperties.
assuming that you can' do any of the suggestions above, and assumeing that the person who you'll be sending messages to actually wants to recieve them, look in the windows directory for an exe file called winpopup. this is a simple messageing pogram that will allow you to chat to each other.