Gang-rape may be a bit extravagant, but that boy and his friends should be prosecuted in some way. The poor girl intended that photo to be viewed by one person, not several.
Wikipedia (the link you provided) defines rape as sexual assualt -- take a look at that link. You will notice that in the third paragraph, Sexting is the FIRST FORM of sexual assault! It is also listed in the Specific Offenses on the right side of the page! Obviously sexters should be prosecuted, no matter their age.
However, I do agree with you -- cellphones should not be allowed in schools.