A temporary altar
Your sacred space need not be in a permanent location. If you have
special items that you like to use for ritual, you can simply store
them in a safe place out of sight. You can bring them out and set up
your altar whenever and wherever you are able. You might even consider
taking them outside to a secluded place if it's too awkward to do so
at home.
Hide in plain sight
You'd be surprised how easy it is to have a permanent altar area
without attracting any attention. You don't need to be overtly Pagan
with the items you choose. A huge chalice with a pentacle engraved on
it isn't the only way to represent Water. You could use seashells or a
pretty flower vase instead. A feather could represent Air, and an
interesting stone for Earth. I do love candles for Fire, and they are
such common items that I doubt anyone would question them.
A few carefully thought-out knick-knacks can represent the elements as
well as the God and Goddess, and nobody need be the wiser.
Think vertically
This is just another way to hide your altar in plain sight, one that
works particularly well if you are short of space. Your altar doesn't
need to be on a shelf or table top. Use the walls. Hang non-Pagan
artwork that represents the things you would like to have on your
altar. A bulletin board would also work, especially if you are living
at school in a dorm room. You won't be able to use these 'items' like
you would physical items on an altar, but images can be powerful
reminders of your spiritual path.
Don't forget: having an altar is important, but not a requirement. If
you are always going to be stressing over having your items found or
getting caught, your mind will never be clear enough for meaningful
rituals anyway. Going without an altar altogether is always an option.