Hi Vitaly,
Each year I go back to my highschool for a similar purpose. I tell them that there is more to software development than just coding: getting requirements, providing support, trying new stuff, drinking coffee, etc. I alway show a bit of code as a glimpse, but is quite offputting to most. On two occasions the Python language was associated to programming by the students. So if you'd like to point them to something concrete, I'd aim for Python. Last year I handed them each a double-side page with an illustrated version of my bio and concrete pointers for the students to get started with programming. I tell them I'm involved with the FSFE and why that matters, but it seems to abstract. My main message is to just try some programming.
Best of luck! Nico