High School physics level only? That's a weird concept for talking about String Theory and Relativity, both of those concepts are well beyond HS physics level.
I suppose one way you can connect the two is that Relativity (General Relativity, that is, a theory of gravity) describes gravity as the consequence of objects with mass distorting the space-time continuum, that is, the union of three dimensions of space, which we are used to, and an extra dimension of time. Objects will interpret these distortions in spacetime as a force--gravity. Today, however, scientists have shown that forces occur because of the exchange of particles. We have been able to prove this by finding the exchanged particles for 3 of the 4 fundamental forces: Electromagnetism, Strong Force and Weak Force. The only force that ISN'T explained by this particle theory of force (The Standard Model) is gravity. Scientists have not been able to find the particle that is exchanged in the gravitational force. String Theory is a modern theory which suggests that if extremely tiny "strings" of energy can explain the exchange of gravity-creating particles between massive objects, we can add Gravity to the list of fundamental forces that follow our Standard Model, and all the fundamental forces can be united under one theory. This would be a great triumph for science. The problem is that if String Theory is correct, then it would imply that the Relativistic view of gravity as distortions in space-time is incorrect. Some scientists believe there is another way to come up with a quantum theory of gravity. Others believe gravity is a separate force from the other three, especially considering its pathetic strength in relation to the others.
I'm sorry, some of that probably isn't exactly HS physics level, but hopefully I've explained the more advanced terms I've used well enough, there's not really a simpler way to explain the relationship between these two ideas. Let me know if you need any clarification.
