Sit them all down and establish some house rules!
If there are that many kids, and in school, they are absolutely old enough to know that there are "walls" which they can not, may not, breech. Just because their mother isn't there does not mean chaos will reign. YOU are the adult here, don't let them run rough shod over you. If you set rules, enforce them, and still show interest and respect to the kids as kids -- look for fun things to do, etc. - then you can expect, even demand their co-operation.
Best of luck to you. Sometimes the pay is NOT worth it!
Jan in VA