This is probably the problem: "We did not prepare the leaf; we just put a small piece of it **between two slides**.".

I suppose you mean you put a leaf fragment between two regular microscope slides, those measuring 26mm * 76mm (1in. * 3in.)?

That won't work. Long story short: microscope optics/objectives are designed with a specific working distance in mind. An average achromat 40/0.65 has a working distance of about 0.6 mm. An average achromat 100/1.25 has a working distance of about 0.2 mm. An average microscope slide has a thickness of about 1 - 1.2 mm... 

In the... well... regular microtechnique, a specimen is put on a slide and covered with a very thin coverslip, having a thickness of around 0.15 - 0.17 mm. 

Also, as others mentioned, using a 25x eyepiece in combination with a 40/0.65 or a 100/1.25 is kind of overkill. With that combination, you enter the realm of "empty magnification". In short: structures appear larger, but no new structures are revealed and the image becomes more and more fuzzy.

And yes: you need to use immersion oil with the 100x objective, but keep in mind this is a very demanding objective to be used, on the part of the microscopist as well as on the part of the slide preparer.