I live with my boyfriend, and his sisters keep going into our room when we leave. I bought a new door knob with a key, but I suspect they’ll try to use a knife to get in. Does anyone know of a secure door knob that can prevent this? Thank you! 
Consider a reinforced door knob or deadbolt. A keyed door knob is a good start, but for extra security, look into reinforced options or adding a deadbolt. You might also consider security cameras for added protection.