Johnstoni in particular and others have given some great advice already on this topic, I think what I can add though is a "case study" of exactly what I did in order to get where I am today as a Keeper
First things first, from the age of 14 I worked both weekend days and every holiday available at a specialist reptile and aquatics shop near my school, I read every book and magazine we had for sale (I didn't have the internet those days!!!). Since I was already working in the kind of place I'd have chosen for Work Exp. at 15, I did mine at a local garden centre and enhanced my plant knowledge instead
Once I'd done my GCSE's I chose to do Biology (B), Chemistry (E!), Geology (A) and English Literature (B) at Sixth Form college.
While I was there I did voluntary work at Rixton Claypits SSSI, we had a free wednesday afternoon for work exp. I was working on habitat restoration for Greater Crested Newts.
At 17 I got the internet for the first time, not only did my knowledgebase expand rapidly (the internet is a wonderful resource so long as you can read critically!) but I setup and ran a website for the customers and reptile keepers that visited my shop and anyone else who wanted information for that matter. I was chosing the species to be stocked and people would drive from Scotland to Manchester to buy from us and receive my advise by this point!
At 18 I went off to Manchester Uni to do Biology and Geology, I commuted home at the weekends in order to continue working in the reptile shop.
I made sure every unit I was able to choose was geared as closely to my interests as possible, freshwater ecology, oil and metal pollution in water, palaeontology (x3), parasitology (x3). My second year dissertation was on the evolution of snakes, my final dissertation was on the evolution of aves.
Between my second and third years we had a field research opportunity, I went to Calahonda in southern Spain and studied the locomotive adaptations of the local house geckos.
During my uni years I also volunteered at the Manchester Museum Vivarium, housing rare and endangered Neotropical frog species primarily. I also co-chaired a new NorthWest Herpetological group, which got as far as meetings within the museum with access to the educational suite before a job opportunity left that endevour hanging.
A month after I Graduated, a friend emailed me with a job opportunity at Durrell, saying it had my name written all over it.
As it was I was experienced, but not nearly enough for a "Senior Keeper- Amphibian Specialist" position lol! I took a chance despite that and applied anyway, at the very least it is good practice.
My application must have read pretty nicely as I was actually initially considered a candidate! but rapidly dropped when the real candidate came on the scene (Whose wealth of experience never ceases to impress me even now!).
The strength of my application and the way it was written ultimately got me a job at a later date.
I was invited instead to do a studentship at the Trust in order to gain just that little bit more practical experience which might help me out next time.
I was a student from Jan 2007 for 3 months, during this time I pushed to do some kind of project, I worked on monitoring Alytes muletensis tadpoles with Visual Implant Elastomer (VIE) marks and a biometrics and then mixing project with Celestus warreni. (turned out biometrics told you nothing but put them in boxes in a row and you could visually determine the sexes hah!).
I'd determined that Durrell was the place for me and that I was in love with Jersey during these 3 months. I would find a way to stay and keep my foot in the door, I visited a local garden centre with a friend (notably the lady that did get the job

) and as we had a look around the pets department I decided I'd offer to take over their aquatics dept and turn it around as it was a sorry sight, obviously I was a bit more subtle about it than that and arranged an interview for the next day and got the job.
Ambition isn't shy.
I worked there as the manager of the aquatics dept for a total of 5 months and achieved a lot, it was a very enjoyable job and the initial pay was more than I am now on at the zoo after 2 years,

but it was simply a means to an end.
So in August 2007 when the curator of herps came to see me with a contract in hand I immediately said yes and started work 2 weeks later.
I've now been working my dream job for 2 years. This is how you could do it, pure determination and a dedication to gainng varied experience.
My A Level choices seem to have been useful for nothing other than getting onto my degree course, I don't think they were a consideration for my job now at all. As for my degree itself, it has stood me in good stead and prepared me well, but it is the least of my achievements in my opinion.
Don't over specialise, keep it broad within Biology and your options are always open
