Question: Whenever I enter my school I'm forbidden to drink energy drinks. I asked them why and they listed lots of reasons why I wasn't allowed.

I asked "why is smoking not forbidden but something so simple as drinking energy drinks is?" and they replied "smoking is a personal decision and drinking energy drinks doesn't just harm you but everyone around you due to the "behaviour" you output under the influence of energy drinks".

Are there any ways I can legally get around this ban?

Answer: 
Unfortunately you cannot, the only solution is to stop at a shop after school finishes and buy an energy drink from there. Then you can drink it as you are not at school anymore.
You could evade it by getting cancer.
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Schools and colleges in the UK are required (by the Equality Act 2010) to provide '[reasonable adjustment](https://www.equalityhumanrights.com/en/advice-and-guidance/what-are-reasonable-adjustments)' for students with disability. Cancer and other debilitating illnesses [deplete energy](http://www.cancerresearchuk.org/about-cancer/coping/physically/diet-problems/managing/putting-on-weight/high-calorie-drinks) and sufferers are often prescribed high energy supplements and [caffeine-rich drinks and injectables](http://www.cancertherapyadvisor.com/cafcit-injection/drug/3107/). Some of these are marketed under brand names but are essentially identical to retail high-caffeine drinks. 

If you got this added to your education plan (typically by showing the college a prescription and a note from doctor explaining why this adjustment needs to be made) the college would be bound by law to allow you to drink them.