How do I Quit My Job Without Burning Too Many Bridges?

I started my current full time job ~7 months ago. I'm sick of being stuck in entry level jobs (I work in technology and I don't have a technology degree) so I'm going back to get a bachelors in CS.

What do I say when I tell my manager that I'm leaving in 3 months to go back to school? I'm unsure of what to say because of the following situations:

• July/August/September are our busiest months so I would be leaving during a time of chaos.
• One of my co-workers is going on maternity leave in the next few months so I'm being trained to do her job while she's out.
• When I interviewed for my current job I told them that I saw myself growing with the company, and assured them that I wasn't interested in a job that aligned with my education (I did some technical training between this job and my last job). The degree i'm going for is more aligned with my previous training than it is my current position and I have a reason why this is the case but I feel like I may look like a liar.
• The last time I quit a job I gave two weeks notice but my job didn't want to me to leave, so the CEO of the old company got my start date at the new place changed. I wound up staying about a 1.5 months longer at my old job than I planned.

Basically I'm looking for a firm way to end my employment with as little bridge burning as possible since there are a bunch of associations between this company and my previous company, and this industry is small.