Student who are going to cheat will cheat no matter what. Having a cell phone does not mean they student will automatically use it to cheat, they are more likely to ask a friend because it is quick and easier. Looking off others papers is more likely to happen than excessive amount of kids cheating with a cell phone.
No, cell phones in school do not worsen cheating. Students have always found ways to cheat. The methods might change with technology, but there will always be the part of the population that cheats, and the part of the population that doesn't. A cell phone is simply a new way to do what has been done for centuries, by cheating in school.