We have a thriving orchestra, run by the peri strings teacher (for a small fee). It's been going for about 12 years. I send out a letter at the beginning of the year which talks about team work, and the importance of commitment, etc, and this gives me an idea of what instruments we'll have and the age of the children involved.
Anyone is welcome, although we stipulate that children should be able to sightread simple music. The orchestra leader has 'Sibelius' and rustles up easier parts when necessary! ATM we have children from Y2 (very easy violin) to Y6.
Obviously the make-up of the group changes every year, some years being better than others. Our current orchestra has about 30 members with violins, recorders (descant/tenor), flutes, clarinets, drums and other percussion and guitars. The Y4a are leading the violins!
It is very useful to have a parent/helper on hand apart from the orchestra/band leader, to help sort problems! (Our helper is a good musician and can help with fingering on clarinets and flutes, rhythm and beat, and just general little niggles.) We also have a TA who plays in the orchestra, and a teacher who plays the piano to keep things going!
I've never had any problems recruiting players - they've always been keen to come (and we offer many house-points to attendees on sunny summer days!) However, we found that lunchtime sessions work far better than the after school sessions as there are always clashes with OOS activities/children wanting to unwind or watch telly instead.
The orchestra is my source of pride at school. I think we achieve an amazing thing with young, fairly inexperienced children. The advantages to the children are manifold: they learn how to sightread, find their place in music, perform in public and appreciate the importance of their part within the composite sound of the orchestra.
I'm sorry, I can't help you with woodwind music, but I certainly recommend that you go ahead!

Good luck
If I can help any more, please ask!