Does leaving Thailand before the 90-day report reset my 90-day period?

Yes it works like Tod says - the expert -

Just make sure you have a re-entry permit !

Whether you even do a 90 day report or not depends on the visa/extension you're on while you're here

Now if you're on a long term visa/extension where you are obligated to do a 90 day report it works like this

If you're in the country for 90 continuous days you are obligated to file a 90 day report. You can file a 90 day report in person from 2 weeks before until 1 week after your due date.

HOWEVER

If you leave the country before the 90th day, the "count down clock" stops when you stamp out and when you return to thailand your NEXT 90 day report would be due 89 days from the date you stamp in, (the day you stamp in is day one) 🙂