Advice for starting HS students on Robotics

[Cornelius Ejimofor]

I am a new HS teacher and I am looking into starting my kids in Robotics in the Spring semester. Could anyone give me advice or compare the different systems out there; in terms of cost, ease to start doing, complexity for a 9th-12th grader to pick up;

I know there is the VEX Robotics system; Arduino; Rasberry Pi; and a lot of others. Please advise if you can or refer me to abetter forum to ask my questions.