There's a reason social media photography more closely resembles this kind of marketing rather more than it resembles art photography, or street photography, or landscapes, or whatever. This is because on social media, people are after a fashion, marketing themselves. A beautiful portrait of myself is totally uninteresting on social media, its marketing value is near zero. What matters is pictures that show me as cool and interesting. This is me at a party. This is me sailing. This is me kissing my hot hot wife.
A blurry phone selfie of me in an interesting context, the supports my narrative of myself, has value.
The only value in a formal portrait or other "good" picture is as a prop for my "I went and had good pictures taken by a professional" story.
I see this on a forum I skim from time to time, quite regularly.
I did a Senior Session with this little bitch, and she went and shared my Professional Images All Over Facebook! With Instagram Filters!!!1!1!11111
Which is to totally miss the point. Your stupid senior session pictures have literally no value to your client outside of the social media universe. You might be able to sell them a couple prints, maybe, if you talk it up right. Mom will probably buy an 8x10. But ultimately, what the client wants is the story, the narrative of being a hip, cool, high school senior, getting professional shots taken because she really is All That.
She's gonna tell that story, ain't nothing you can do about it. You can blather on about no digital rights and no editing and blah blah until you're blue in the face. One of two things will happen: the client will respect the contract, and get 0 value (and probably walk away), OR the client will ignore the contract.
She doesn't want some shitty photoshopped portrait of herself looking like a portly Barbie doll holding a guitar, she wants a Story about how she is All That, and she wants to tell that story on social media.
So, stop selling her prints, and start selling her a marketing campaign. Sell her limited digital rights. Sell her an hour of time where you'll work with her to instagram the shit out of her session's pictures. Don't sell her crappy pictures, sell her your alleged story-telling skills. You can tell a story, right? I mean, it says so on your web site, it says you have a passion for telling people's stories with your camera, or some similar bullshit. So, you can actually do this? Right?
Protip: BMW executive don't hire people to take pictures of their cars because they secretly crave airbrushed 8x10 prints of the cars to hang in their bedrooms.