So, you want to implement RtI at the secondary level? 

There are five components of implementing a RtI framework for academics or social/behavior

1) Commitment
2) Leadership
3) Assessment
4) Instruction
5) Problem-Solving/Organization

But, wait.  It probably feels like an insurmountable and undefined task to "begin" RtI implementation in a secondary setting.  Here are some guiding tips for where to start: 

"If you want to reshape high school, start by changing ninth grade.” (Donegan, 2008)

“Reform initiatives should single out ninth graders with academic deficits for special support . . . given that success or failure in ninth grade is a pivotal indicator of whether or not a student drops out.” (Quint, 2008)

Chicago Public Schools: “On track indicator” - ninth graders are 3.5 times more likely to graduate when they: (1.) Pass 4 out of 5 classes (2.) Earn no more than 1 F (Deschler , Adolescent Literacy - August 5, 2008)

Look at your data. Yes, your data (not your schedule, yet).   What are kids telling you they need to spend more time doing? Reading? Writing? Math? Engaging in learning? Using prosocial skills?
 
"The Linchpin Year” by Billie Donegan in Educational Leadership, May 2008 (Vol. 65, #8, p. 54-56)

“Lessons from Leading Models” by Janet Quint in Educational Leadership, May 2008 (Vol. 65, #8, p. 64-68);