For users here is a "bulk upload" feature in the dotlrn user adminstration lets you upload from a CSV (excel) file.
There is also some integration that has been created with the IMS Enterprise spec if your school system happens to support that (probably unlikely for a high school).
Do you happen to use LDAP in your school? There is also integration written for that.
Are you from the US? What software is your current class lists in?