As Denise pointed out, it can be tough at times to tell if some samples of petrified wood are, in fact, petrified wood. There are also materials that can mimic the appearance of petrified wood - but which aren't...
If evidence of the wood's original cellular structure or rings is still present it is usually fairly easy to see that it is petrified wood. You shouldn't have any trouble with those well-preserved examples. But often all one has to work with are small chips that don't show the rings and which do not preserve the original cell structure. Then it is a matter of experience: Having seen enough petrified wood in better forms to be able to tell that what you are looking at is a chip of something that - if you had a large enough piece - you would see the tell-tale signs in. There is no shortcut to obtaining that experience.
What your students are likely to have will depend on where you are located. If you are in the American Southwest, or certain other of the Western states, where petrified wood is plentiful, then the odds are the students have been picking it up and will already know what they have (at least for the most part.) If you are in an area of the country where petrified wood is scarce or absent, then the odds are what the students will have will be other things that they just *think* might be pterified wood - or, if they have petrified wood, they got it while on vacations or as gifts from relatives and friends. So ask each student where they got the rock they are showing you. That's your first big clue as to whether or not it is likely to be petrified wood. If it came from somewhere in the Southwest, it may be; but if it came from - say - the Adirondaks, not a chance... :~}
Taking photos and posting them at rock and mineral eBBs (such as McRocks or Mindat.org.) might help; but I suspect that photos of any petrified would that can IDed as such from an image will be of samples you can ID yourself - they'll have the tell-tale characteristics. But chips that do not exhibit cell structure or rings will probably be just as difficult for experienced rockhounds to ID positively as they would be for you to do so. They could be chips of petrified wood - or just pieces of chalcedony or agate... They would have to be seen in the context of the locality they came from to tell for sure.
Bottom line? You'll get a few pieces that you'll be able to say: "Oh yes - see the cell structure (and/or rings)? That indicates it is, indeed, petrified wood." A few of the pieces you may look at and say: "Sorry, no." (Not the right mineral, not from a place where petrified wood is found, etc...) The rest of the samples you'll end up shrugging your soulders over and saying: "Maybe - but I can't really tell..." That's pretty much par for the course for most of us.
PS: I had the good fortune a few years ago to visit the Peabody Museum at Yale and spend a couple days poking about the back room where they store mineral specimens that they don't have room to exhibit. They have a case of "pseudo-fossils" that includes several items that were identified as petrified wood by professionals - but which weren't petrified wood. Even the pros get, uhm, "stumped" once in a while! :~}