How do I decline this job politely and keep the opportunity to work there, maybe next semester (or even in 2 semesters which is approximately 8 months)?
Do not decline anything! Respond to them and state the date of your availability after this first semester is over. That's it.
And if you're willing to work there next semester, do not mention that you'd be willing to work there two semesters from now, or they're going to assume that next semester is not a good time for you, but that you're just not assertive enough to speak your mind.
And building on the previous message written by Ertai87, this is what I'd write.
Hi (recruiter's name),
Thank you for reaching out. I'm very excited about this opportunity.
Unfortunately, I can now only start on (insert date here).
Would that date still work for your company? Since I had not heard back from my initial application two months ago, I'm afraid I've already taken on a full course load since then.
If my availability works for you, I'm available for a phone screen on (insert days and times). My number is 555-555-5555
Of course, this message is assuming that you're applying to some kind of part-time internship program. If you're applying for a different type of job, or if I'm missing some crucial piece of information, this message may need to be changed accordingly.