You may have heard in the news recently about the "heartbleed" vulnerability that has effected many secure websites this past week. While it is unlikely that your school gmail account was compromised, it is still necessary to change your password as a precaution since gmail was one of the vulnerable sites.
It is also wise to change your password on any personal email account, as well as sites like FaceBook. In most cases, companies are issuing notices to their customers if their accounts may have been compromised, so you may not have to change your password "everywhere".
In any case, it is a good idea to change your password regularly as part of good web "hygiene". Here is a guide to choosing a good passwords that are easy to remember.
Note that students will get new passwords at the beginning of every school year, but will not have their passwords changed at this time.