That arm has a scar that's left after you've been vaccinated against Smallpox.
Most of every fricking mexican my age got the vaccine at some point in their life, either by getting it at an hospital at whatever point, or forced at school (with parental permission via: "Hey you dumb kid, give this consent to your tutor. If your tutor signs it, you get vaccinated.")... I still remember being in 5th grade and one day I played all day because it was "Smallpox day" and when I showed my scar on my shoulder I got a: "Ah, fine, do whatever" and witnessed many children getting vaccinated while enjoying the smell of alcohol and the lamentations of kids crying.
THEN at middleshool you start to get options with vaccine days, most famous one is against Papilloma, mostly kinda forced on girls age 10+ (tutors still can decide over the child, and you can miss that day at your school) which I believe is a very good vaccine for girls, as the vaccine is linked to preventing (as in you become 95% immune to develop) various types of Cancer.
- I'm aware old people and military folks around the world probably has the scar too, but it's more common in Latin América.