We say :

The range and quality of headphones available is huge! For our education customers, we recommend the PreSonus HD7 as the best entry level point for headphones suitable for the classroom environment. We have sold these headphones to many schools over the last few years and have had good feedback from teachers regarding their quality and robustness. They are supplied with both a mini-jack and standard jack adapter. 

We would strongly urge teachers to not purchase cheap headphones. It is a false economy. As with cables and leads, if you buy cheap you buy twice (at least)!

PreSonus say :

Whether you’re monitoring, tracking, mixing, or just listening for pleasure, you can expect impressive sonic performance from these professional-quality headphones. They deliver natural, warm, clear sound and take advantage of a patented, semi-open sound chamber to deliver exceptionally deep low frequencies. You’ll enjoy accurate midrange and extended treble response that let you pick out the fine details of a mix or track, along with balanced, yet powerful bass punch.