Hey!
I would try not to worry too much about having a huge or outstanding achievement - firms will usually care much more about how you talk about something than how objectively impressive it sounds.
A lot of good answers are often quite ordinary things that are explained really well. Therefore, I'd say the key part is choosing something where you can clearly explain:
- What the situation was
- What challenges you faced
- What you specifically did
- What you learned/gained from it (+ how this has impacted how you approach similar situations)
I wouldn't say it needs to be recent or legal-related, and a high school achievement is completely fine if it is the best example you have and you can reflect on it well. I always used to discuss my grade in a public speaking module at university as my 'greatest achievement', as I could reflect on it well and show why it mattered to me and my own development.
You could even use things such as balancing work with studies, leading a society or team project, caring responsibilities, or improving in a subject that you initially struggled with. What matters most is that you choose something that you are genuinely proud of, and can discuss how you approached the challenges.
For the "challenges" part, they are usually looking for resilience and self-awareness, rather than perfection. Therefore, don't be afraid to talk honestly about any difficulties or setbacks that you encountered, provided that you then focus on how you dealt with them.
Best of luck!!