Really depends on a number of factors, like...
- dxes
- behavior in school
- what specific things the aide is expected to do
- age/grade
1:1 is the hardest to get - because it's the most expensive. I've seen it most often with severe medical challenges (quad, severe CP, etc.), or with the more serious pervasive developmental disorders - Downs, at least moderately-severe Autism Spectrum Disorders (ASD), etc.
In-class shared aide is easier - we often have one aide working with 5 or 6 students who have challenges - this aide takes notes for anyone who needs them to get a copy (including the kid who was sick yesterday...), helps explain assignments, walk through addiitonal examples, etc.... all while keeping an eye on behavior. That's just an example - and this is obviosly older kids, as grade 1 kids don't take notes!