How many static IPs do I really need?

We are an alternative High school in the Chicago area. We are using a T1 and a DSL line. However, because of the NCLB laws we have to add additional computers to the class rooms instead of just the labs. We are thinking of adding Comcast cable to an already overworked network. The question is, since, we will be adding aprox 60 computers (2 to a classroom) to the cable line, would static IP addresses (at $35 a month) be of any use or would dynamic ips suffice?

As of this moment the t1 has apx 70 computers. Each computer has its own static IP address, going to 1 Gateway address. The DSL is running aprx 25 computers, almost all are running as dynamic, except the administrative computers which I set as static going to 1 gateway address. I will be adding the sixty or so computers within the the year, about 5 to 10 a month.