M.H.
You can buy paint that dries into a chalkboard. You can then write on it with colored chalks. My suggestion would be to tape off one wall of her room. Paint it with the chalkboard paint and buy her some great chalks. This must be her need to express herself. At age 12 the emotional roller coaster is just beginning. If this is her outlet... let her have it!
You can also use the paint on boards to hang about the house in places she needs to write. You could even decorate around them with other colors of paints, ribbons and bric a brac. Make it a group project and try to find out why she needs this!
Good Luck!