Sorry for the pending religion/history lesson.
Padua is an independent in nature, if it wasn't tuition wouldn't be so high. It might be tied because it's basically a parish school, but it does recruit from the National Assoc of Independent Schools.
The others need the Diocese to survive. The diocese needs Ursuline, Salesianum, Padua and Archmere to survive. Working in a small Diocesan high school now I joke that the Diocese of Wilmington loved the independent schools when we won state titles, to the degree that the Bishop showed up to celebrate the Triple Crown in Fall 2006, he even lobbied for us to have a day off.
Dovetail is correct, the millions lost in the scandals will never be recovered and had a crushing effect on the overall Diocesan coffers.
When you cannot pay and cannot give a return on investment like you once did, many will not pay. People that were truly Catholic had a hard time buying into St. Mark's. When you look around you have the Salesians, Ursulines, Dominican Panda Friars, Norbertine Auks and Benedictine Vikings. St. Mark's always struggled with a charism and Catholic identity. It makes a difference, especially in raising funds. The two most successful Catholic schools in the area, Salesianum and Malvern have deep fundraising pools, guided by strong charism. If St. Mark's could link with the Jesuits or Xaverians they'd be right back at competing with Salesianum.
As for the general decline of St. Mark's, it's bad and really pitiful. I spoke with admin at Salesianum three years ago and said that this decline was bad for the rivalry. I went back for a wrestling match this year, what a difference a few years makes. There are a variety of reasons, I don't see them improving anytime soon, and if the Diocese considers contraction, expect St. Mark's to shut down, St. E has done a great job of maintaining enrollment at or around 700 for a long time. St. Mark's has been bleeding kids since 2008. The St. Mark's property has much more land value to draw in a charter corporation or Wilmington U. Satellite Campus.
I'd considered applying for St. Mark's principal job as recently as last year because when they aren't good the rest of the state isn't as good, and they need leaders that understand the value St. Mark's brings to the community.