I would say pretty low. I do know of two people who met in 7th grade and got married after school (high school) but, I would think that you meet to many other people along the way and find differences that weren't there which could make you break up. You also grow up and might not like each other later.
That doesn't mean it can't happen but I believe it is pretty low.
About .002% of middle school relationships last until marriage.
Zero percent.
79% of high school sweet hearts will get married
A number of how many couples have married has yet to be released.
There are 12 same-sex couples that have married in Rabai.
There are 11 same-sex couples that have married in Kaloleni.
There are 10 same-sex couples that have married in Ganze.
There are 12 same-sex couples that have married in Wundanyi.
There are 9 same-sex couples that have married in Balambala.
2 same-sex couples married in Wahkiakum County in 2013. No same-sex couples married there in 2012.
At least 28 same-sex couples have married in Pará. 28 couples were married there on June 28, 2012.
There are 12 same-sex couples that have married in Lamu East.
Unfortunately, no same sex couples can be married in Harrisonburg, VA.
20 same-sex couples have married in Colombia as of March 2014.