Such a great cover for Teen Titans 16 (July/Aug 68), and a fun story by Haney and Cardy to go with it.
The tale centres on a boy who has become the laughingstock of his town, after he tells a story of finding a gateway to another dimension, and invading aliens who are plotting conquest. The students all mock him, and the principal is insisting that he be taken away for treatment. He calls in the Titans, and it seems that Robin, Aqualad, Wonder Girl and Kid Flash are the only ones who actually follow in his steps, climb the water tower, and depress the button that opens the gateway.
The aliens from that world have a few entrances, as it turns out, in various places in and around the school. They have big plans to kidnap and replace the students with doubles, as a start to their invasion. The Teen Titans fight the aliens both in their world and in the real one, which have some identical locations.
The principal turns out to be the leader of the aliens, one of those taken and replaced, it seems.
A good, solid, straightforward action story, and popular enough that these aliens made a return the following year.