Please add a comment with the specific model of Janome Schoolmate sewing machine.
Fpr the Janome Schoolmate S-3015, make sure that the needle is fully up and raise the presser foot. Then press the red button on the back of the presser foot holder to have that foot drop off. Set the pressure dial to 3 before attaching the new presser foot. For snap-on feet, align the pin of the presser foot (the horizontal bar in the middle of the foot) under the presser foot holder. Lower the presser foot holder (using the presser foot height lever) until it locks onto the foot.
For the zipper foot, you need to fit the rear pin on the foot into groove at the back of the presser foot holder. Then lower the presser foot holder onto the zipper foot.
These are shown on page 11 (14 of the PDF) of the manual. You can download the manual here: http://janome.com/en/machines/sewing/schoolmate-s-3015/
. The direct link to the manual is http://janome.com/siteassets/support/manuals/school-models/schoolmate3015.pdf
For the S-7330, you need to again raise the needle and the presser foot. Press the lever towards (it moves slightly up, not down) the presser foot holder. Then line up the pin on the foot under the presser foot holder and lower the presser foot holder onto the foot so that it locks. This is also in that sewing machine's manual, here: http://janome.com/siteassets/support/manuals/school-models/inst-book-s-7330esf.pdf
. See page 14 (18 of the PDF).
I hope this helps.