Henry Purcell (1659 - 1695) is often considered the creator of the English style of baroque music. I arranged his sweet Air in B minor for our junior orchestra.
Some of the parts are a bit technically demanding, and may work better for older students, although the Violin 2 / Viola and Cello 2 parts are quite easy to play. This would make a nice piece for an orchestra with players of varying ages and abilities.
Complete parts are available in this collection of baroque pieces at Sheet Music Plus: A Baroque Journey for String Orchestra.