If it's not 'loud' then you probably haven't got much you can do. There are laws here in the UK about noise at night (11pm onwards) but not much for the day.
I can see it panning out like this:
1) you complain to council
2) they write a letter to the school (which may be ignored)
3) you complain again
4) The council will send out someone out with a decibel meter, stand in your garden and measure the noise level, which, by the way you describe it, will barely register
5) You live with it.
I can tell you, from experience, that it has to be pretty loud in your boundarys to be classed as a problem. I had complaints once years ago at a space I used to rehearse in. The Guy came to measure and he said something like this "Hmmm, 76 decibels, you've got room to spare, carry on boys!". Now, the limits may have changed but if it is one of those 'in the background' sounds, you're screwed.
I'd suggest you go and have a (polite) word about a compromise before you get all legal.