Marketing is all about putting your message out where your target audience will see it. Do they use Instagram? Make sure you have one. Do they use Facebook? Get that one too. You can duplicate posts between platforms but do keep in mind that people use platforms for different things. Images with great stories work better on Facebook than images alone.
Try your hand at digital advertising. Facebook will let you refine a target audience based on demographics that you get to select and then you can cap a max budget for the campaign.
The absolute cheapest way to get new clients is in repeat business and referrals. What are you doing currently to encourage repeat business? What are you doing to encourage referrals? Do you find reasons to reach back out to clients? (For example, let's say you photographed an Engagement Session. When was the wedding? Did you send a card? Were they planning on kids? Is it in your calendar to check in with them about family portraits?)
With high school seniors in particular, photographing someone in a clique is a great way to get the rest of the gang knocking at your door. Make sure they have a good time during the shoot, love their images, and make sure they have a shareable version ASAP (something they can post on social media bragging).