I'm a senior at my school. A socially-awkward prefect, actually. Because of my shyness, I often ignore my juniors' misbehaviours, leading them to be spoilt. Also, they don't really care about what we seniors say because most of us are shy too. Our teachers are mad with the juniors because of their uncleanliness, rudeness and poor academic results, to name a few. So, how can I(we) scold them, just for the sake of the lesson, without annoying them?
A lot of this depends on the relationship you have with your juniors. I'll approach this from the US perspective.
If your relationship is based solely on power, then there may not be a lot of respect there. That makes it harder to make the scolding be effective.
I assume you can't assign discipline to the juniors (back in grade school, it was clapping out erasers. You'd go outside and clap erasers to clean the chalk dust out. It was dusty, dry, and not pleasant. No one does that any more...)
I'd start by focusing on one thing. Pick the most egregious one and deal with that. Compare it to something relevant that really matters to them. "Adults act more politely with others than you are right now. I'm, quite frankly, disappointed in this." "How often do you see [famous person that they idolize] in the news for acting like this? Sure, they may do it once in a while, but not all the time - which is what I see here. I just don't imagine this happening with him/her." "You would talk to your mother that way? Really?" That may make the issue relatable to them.
Instead of scolding them try to appreciate the people who behave polite/clean/score good marks. You can offer chocolates/cookies to them.This will make the other children to behave good. If you are studying in a school, then I am sure your juniors will be fond of chocolates/cookies. You can also ask their teaching staff to give bonus points to them for their neat and polite behavior. By this way, you can make your juniors behave good without scolding them.
A general look on such a situation. Scolding tells them you are not happy with them. If they don't care what others think, this will have no effect. Worse - it will have a negative effect as it decreases their respect for you.
What you need is something to make them not happy with themselves.
How? This depends on your role in this school. I don't understand this junior-senior relationship and why teachers are mad about them but you should correct this. So it may be the best first step to talk to your teachers about that.