There is no simple answer. I have never oiled my clarinet bores, and I have never had a crack. However, some people have a body chemistry that causes the clarinet bore to dry out considerably. If your instrument has a very dry bore, it is a good suggestion to oil it. Do this only during cold months when your heating system dries the air or if you reside in a very dry local weather. To oil the bore, place a few drops of bore oil on an old swab and pull the swab throughout the device. The critical thing is that if you do it once, you have to then do it continually — once every two weeks or so during the winter months in cold climates, or more often in very dry climates. Do not use the rest other than the bore oil you discover in music stores, that is formulated from light mineral oils that won't turn rancid. Oiling the bore will not affect your clarinet’s tone or reaction. Finally, be certain that your scholars never do any of the following to abuse their clarinets: stand it on the ground by the bell unless, of course, they've got a clarinet peg; lay it on a music stand; leave it on an unstable chair or lay it on the ground; carry it throughout the halls with no regard to overlaying the mouthpiece from hitting the walls or their friends; leave the mouthpiece cap off while waiting in line to take an all state audition; leave it unattended — in or out of the case. Teach every student to look after and give protection to his or her clarinet.