does it start as early as the first week in March?

That is generally a little early for March Spring Break. Some public high schools ion Northeast US do a week in February (to coincide with Presidents' Day) and then another week in April. Most colleges do some time off in March (as do many HS in the rest of the country) - but not usually as early as first week in March.

Thanks - we've got flights to Nassau and as I said in another post, I'm now wondering if it will be a crazy party scene while we're there (not what we are looking for).

High peaks are the last week in Fe18-26) for US school vacation week and the last week in March for many colleges -- with the in-btween time also high season.

Has anyone been to Nassau during the first week of March and if so, how full of college kids is it?

Where are you staying in Nassau? If you are staying at an upscale hotel, you won't have college kids. We stayed at the Atlantis and I would be shocked if someone without a really good job could afford to stay there. I think colleges are varied on when they have off. All of March and early April will be spring break time for colleges. The younger kids set will most likely have the week before or after Easter holiday for a vacation.

You have posted this question at least three different places on this board which indiciates you are quite worried. You have already booked your trip, so instead of worring about something over which you have no control, take a deep breath, realize you can have a great time no matter who else is there, and relax while planning your wonderful trip. I read on another post that you are staying at the British Colonial Hotel. I doubt that is the type of place that would appeal to "spring breakers".

Last Years Spring Break ran from Feb 15 to April 11th with nearly every week being a party week.
Most all of the big hotels, such as Radisson, Wyndham, Nassau Beach and ATLANTIS were inundated with spring breakers (yes, even Atlantis since there are plenty of affordable party packages with 4 to a room).
The smaller spring break hotels like Holiday Inn, Quality, Town Hotel, Dillets etc will be overflowing

You can find some peace at places like British Colonial Hilton or Orange Hill, as well as some of the timeshares and private villas.
Not sure yet what the policy will be at RIU for spring break, but since it's just next door to Atlantis, the beach will be crowded.

I'm with schmerl. You sound like you have already booked your trip. So go and enjoy. The Hilton looked like a very nice hotel and I am sure it will be relaxing. If you are that worried, pay the penalty for airline, and hotel, etc.. and change it to late April. I would just go and have fun when you originally planned.

It can also sometimes be kind of cool the earlier in the year you go. Think goosebumps coming out of the water. Late April is a terrific time to go.