Mail merging students into separate groups

I have a list of students on Excel, about 1000. These students are from 100 schools. So the data looks like this:

NAME - SCHOOL NAME
Joe Smith - Polk High School
Jane Blogs - Polk High School
Sarah Jones - Polk High School
Tim Stevens - Roley High School
Hugh Heffner - Roley High School
etc etc

I need to merge this data into a separate letter for each student. Then each school will get the letters for their students.

What I have had to do in the past was mail merge all 1000 students into a huge Word document sorted by school name. Convert to PDF. So then I had 1000 pages in a PDF document that was sorted by school. Then I would go through and extract the letters for each school into its own PDF file and email it to the associated school.

Separating the document into schools was really time consuming. The merging was easy. Is there a faster way? I'm using Office 2007.

Cheers