Sorry if I sound somewhat surprised, but... you're kidding, right?
Almost all bigger, and dozens of smaller, airlines offer youth- and student-fares, and there are several companies specialising in this field:
Look up their websites (or one of their agencies) and you'll get all the info you need.
As for DL
ignoring the people who use discounted fares with their Simplifares structure: back when I worked in a regular travel agency (I'm now employed with one of the above mentioned companies), I was always surprised when people came in and asked about getting the cheapest fare and going on to say that they wanted a student fare: to that, I frequently replied that they would actually have to make up their minds between the two, because, often enough, a special offer will be cheaper than a student fare.
Student fares are usually a lot cheaper if you need a long maximum stay, or low rebooking fees, or the possibility of having a ticket with an open return - but if you're just thinking about flying somewhere for the weekend or for 1 to 3 weeks, the normal special fares will, quite often, be cheaper.
Just as an example: If a regular fare on XX airlines for a specific flight is $500, with a student fare of $350 being offered, while DL
's new structure offers the same for $250 with no further student discount being offered, are they really ignoring students, or are they simply offering an even better fare to everyone?