I left university at 19, and I don't regret it. A couple of caveats.
I posted elsewhere regarding paperwork. Make sure the offer is what it sounds like. If you give up your degree and end up getting shafted, it'll not be fun.
Secondly, it depends on your school, frankly. If you're in a top tier school, your odds of long term success by staying and building a personal network may very well be great enough to not be worth risking even if this startup seems very attractive.
On the other hand, if your schools CS program is relatively unknown, then it frankly doesn't matter much. If you see the need for a degree later, ask yourself: Can you find somewhere in your current schools league to finish a degree? The reality is that outside the top tier of recognised names, nobody cares much what school is on your degree unless the interviewer/hiring manager happened to study there. In that case, consider the "risk" of not being able to re-enter your current program as pretty minimal: It will cost you some time, and maybe a bit of money if you belatedly decide you need to finish your degree, but it won't cost you a high value degree.
I'd be inclined to recommend to take the year for the value of the experience, but think long and hard about whether to go on after that year. It greatly depends on the success criteria the founders want for you to commit to stay on after the year. If the success criteria means you'll be able to put a good title in a (by then) high value business on your resume, nobody might ever ask about university again. But if their success criteria means you'll stay on in a ok but unknown business that's not growing very much, you should be much more careful.
For my part, I never went back. My first startup didn't do great, but it taught me more about business, sales, accounting as well as technical subjects than I ever thought I could learn in that short a time (or ever had any desire to learn, in the case of busines, sales and accounting...). It lead directly to getting high value (at the time) contracting jobs, as well as being hired into a senior developer job at 23 (at the recommendation of the lawyer of the guy that invested in our first business) that got me poached into a director of technology job at 24, and led to me meeting the people I co-founded my first VC funded startup with at 25.
At 28, I went back and did a degree via distance learning, and I now hold a MSc. But at the time, the only reason I did so was that the job market was weak, and I basically wanted to dot the i's and ensure I had a degree to put on my resume "just in case". In the years until I was 28, my lack of a degree never came up as I moved from place to place by virtue of personal relationships and never went through any formal interviews. In the years since it's mainly come up as a curiosity ("so how come you finished your degree after you'd gotten that much experience?" "oh, you know, just wanted to make sure I win the keyword bingo with recruiters in times when the job market is tough", pretty much, and that's it).
Of course, that worked because my first startup set me on a trajectory that gave me lots of experience quickly. If you do leave school, the one key thing to make sure of is to set firm goals for yourself for where you want to get and make sure you do go back to school the moment lack of that degree becomes a hindrance to getting where you want. E.g. if the startup is doing ok but not great, you need to think long and hard about whether or not it will give you enough experience and clout to compensate for lack of a degree.