I miss you- The person is not with you; you might be on the phone or writing when you use this. It could also be used to describe a pattern of missing the person- they go away regularly and you always miss them, in which case you could be telling the to their face.
I missed you- this just refers to the past. You could be talking abouty a long time ago, or when they have just arrived.
I've missed you- I'd use this when the person arrives.
- For Teachers