If you modify a commercial tune to become a ring tone does this infringe copyright?
I would have thought to but it seems it might not. Apparently ring tones are NOT considered derivative works and (as long as you legally own the original track) you can create and use your own ring tones. NOTE that you cannot sell (or even give away) these commercially.
You cannot use downloaded music though.
Note that nobody seems willing to ’stand by’ this advice. I place it here to get you thinking but you should make proper legal investigations should you wish to do this (due diligence). It isn’t an area I’d go into with pupils unless you use music that they compose.