The most important thing for a firesteel striker is that it is hard and has a 'crisp', although not necessarily sharp edge. Stainless steel knives tend not to work quite so well as plain carbon steel ones.
In general when you use the back of a knife as a firesteel it needs to be ground with a well defined square edge to the spine.
Carbon steel files work extremely well and emery paper is pretty good (although a bit less durable than other alternatives) so the emery boards sold for filing nails should be a good option.
Short lengths of hacksaw blades can be effective as well.
Using an abrasive rather than a blade also has the advantage of being a bit safer for teaching.
Emery paper or boards, tend to produce more sparks and are, in some ways, easier to use than a scraper type striker (eg the back of a knife). The difference is that emery does not 'throw' big fat sparks in the same way as a scraper so a slightly different technique is required. One good method is to wrap a strip of tinder around the tip if the rod and hold it in place by gripping it near the end.