Tears should be allowed but depends where they are. Anyway yeah they can easily be torn (i got a hole the size of a baseball once when i tried to get comfortable). Not everyone can afford to mend the pants over and over and over. Anyway its not like someone is going to look at the leg and say ooh thats hot or something.
Sometimes tears occur on jeans by accident, and cannot really be said to the fault of the student. If a student is poor they should especially not be held to fault, because replacement or repair of clothes could be too expensive to be practical. Tears in jeans can also be expression, and do not necessarily need to be located near risky places on the body, such as the lower leg.
Schools should not allowed tears in jeans, becuase it against their clothing policy in most schools. Also to keep in mind that the how people dress is how they act. These kinds of clothing would have wrong impact kids. One of the reason why schools have uniform is to build up that white collar personality in their students.