No worries, here to help!
Manage your time and start early - I cannot stress this enough. Aim to start seminar preparation a week/several days in advance and try to finish a day or two before the seminar. There will be times when you're really busy and you finish it later than you expect, so this approach allows for that.
To keep reading time low, skim-ready sections of judgments/articles that aren't as important. When you take notes, try to make them a summary and don't make them too detailed - this will help a lot when it comes to revision too! If you're reading an article then really try to hone in on what the author's stance/argument is. For judgements, make sure you understand the outcome of the case and the reasons why that was the outcome (i.e. ask yourself how the relevant legal principles were interpreted and which facts influenced the outcome).
That's a great question, and I'm honestly not sure what the answer is! I had an amazing time at uni and I'm really happy with how things have turned out, so perhaps I wouldn't change anything at all.
You learn as you go. Of course it's different to secondary school - there's no mandatory attendance, no school uniform, you're in charge of your own leaning, you're (usually) living away from home, etc. I went to a college to do my A Levels (my school did not have a sixth form) and the college operated in a similar way, so that was an excellent bridging experience which prepared me very well for university.
University is a lot more about developing critical thinking and analytical skills. There is memorisation involved for closed book exams (e.g. knowing the legal principles, case names, etc.) but the high marks always come from how well you can analyse and apply the rules. For essay questions, particularly for open book, it's all about the quality of your argument, the structure you adopt, etc.
Given you asked about secondary school, I'm going to assume you're at the pre-university stage and so will not go into any specific details which relate to the application stages. My advice to those at the pre-uni stage is to firstly just focus on getting onto the law degree (if that's what you want to do!), and to ensure you get involved with extra-curricular activities to develop your soft skills and give yourself examples to talk about in applications/interviews. In addition, keep up with the news and develop your awareness of business and finance matters, particularly if you are looking to go into commercial law. Every law firm is essentially a legal business (it sells time and expertise), and firms love it when applicants properly grasp this.
You too!