Check out
https://plugins4free.com/ where there are LOADS of free plugins. I love the site, and you could find things other than samplers (like synthesizers) to use.
Me, uh, I haven't tried it much myself, but there's Viena the soundfont editor. On
http://www.synthfont.com/Downloads.html if you scroll down to the
Viena and Viena64 (1.192) - the free SoundFont editor section, there is a non-admin install available, which is what I have. I used it once for converting GM to sf2. It didn't work that well, but that is probably because GM is an unheard-of extension. It probably works better for sample work and sf2 creation.
If you could get the hang of it, you can map the sounds to different keys, save it as a soundfont, and then use the soundfont itself in LMMS through SF2 plugin.
Also I wouldn't recommend using primarily school computers as one day you may have to return them.
Let us know what works to you, so we can remember it and use it when suggesting to others. All the best!
