it's 'forever' some places.
in our state expulsions can be permanant and a parent has to petition to get a child readmitted. they can try and enroll them at another school district but since it requires the district you live in and the district you want your kid to attend to both agree on allowing the child to attend outside their assigned (geographical) district, and you can't technicaly get a transfer from a district you're expelled from-it can be next to impossible to get around it that way.
the state has to provide education so different districts have different things in place for expelled students-it might be continuation school, on-line classes or something where the student meets with a teacher for just a couple of hours a week to receive the next weeks lessons, review what they've turned in previously, do testing (basicly you're expected to homeschool with public school oversite/free curriculum).
i don't know how it's set up here, but the school district we used to live in (in california) had certain offenses that carried a MINIMUM one calendar year expulsion, so if a kid did one of those the last week of the school year they were expelled until at minimum the last week of the following school year (and with those types of expullsions the school could require a multi step plan that had to be documented as followed during the period of expulsion in order to even submit an petition for re-admission, and if another school saw that type of expulsion on your record you could'nt get enrolled until the whole issue was cleared up).
the private school my kids attended in california (and actualy their affiliates all across the u.s.) started catching on when they ended up with disciplinary issues on kids that had enrolled after the begining of the school year (and it was'nt due to a move). they realized that parents were enrolling expelled kids. after a few realy nasty situations the education division of all the schools implemented a policy where a child could'nt be enrolled and begin attending until the principal made verbal contact with the principal at the kid's last school of records AND received the kid's paper records FROM that school (had some parents bring in supposed records that conveniently had missing information pertaining to disciplinary history

).