I haven't done one of these posts in a couple years, so I decided to do an updated one with the updated grades and the addition of new contemporary characters!
Lindsey- Regardless of when her birthday is, she's out of college. Unless she wants to be a doctor.
Kailey- Same as Lindsey.
Marisol- Same as Lindsey.
Jess- Same as Lindsey.
Nicki- In Thanks to Nicki, Nicki enters fifth grade during the 2007-08 school year, so that means that she is in senior year of college.
Mia- In Mia (which starts in October 2008), Mia states that she is 10 years old. The New York (where Mia lives) cutoff date for kindergarten is December 1st (which makes her a fifth-grader in Mia,) so Mia started kindergarten in the 2002-03 school year. Therefore, she is in her junior year of college this year.
Chrissa, Sonali and Gwen- All three of them are starting fifth grade in the 2009-10 school year (proof: Chrissa Stands Strong: The Book), so they started kindergarten in the 2003-04 school year, which makes them sophomores in college this year.
Lanie- Lanie is about to enter fifth grade in the 2010-11 school year, and that means Lanie started kindergarten in the 2004-05 school year. Simple math shows that Lanie is a freshman in college now.
Kanani- Since Kanani's grade isn't mentioned (the whole series takes place in summer 2011) and I know her age is 10 in the books, I looked up the Hawaii cutoff date for kindergarten (July 31) and that's when I knew that she would enter 5th in 2011-12. Simple math proved that Kanani is a senior in high school this year.
McKenna- McKenna is a fourth grader during her whole series that is in the 2011-2012 school year (according to her movie), so she is a junior in high school.
Saige- Saige is in fourth grade during the 2013-14 school year, so she's a freshman.
Isabelle- Isabelle is in fourth grade during the 2014-15 school year (like me!), so she is an eighth grader.
Grace- Grace was in fourth grade during the 2015-16 school year, so she is going to be a seventh grader next year.
Lea- Lea was in fifth grade during the 2015-16 school year, so she's in eighth grade.
Gabriela- Gabriela Speaks Out confirms that Gabriela is a seventh grader now.
Tenney- It can be drawn from Tenney's series that Tenney will be in 8th grade this year.
Logan- Logan will be in 10th or 11th this year, because he is 14 in Tenney in the Key of Friendship (March-April 2017) and the cutoff date for Nashville schools when Logan joined kindergarten was September 30th.
Z- Z is in ninth or tenth grade this year, because she is 13 in The Real Z (April-May 2017) and the cutoff date for Seattle schools are August 31.
Luciana- Luciana is in sixth grade, because she is 12 now and the cutoff is September 30th.