In reply to George B01: There's some great advice in this post George, and as you can see a lot of people have said the same thing. I'd agree with them. From the grades you're climbing, and the amount of climbing you do currently, as well as your age, supplementing your climbing with a fingerboard will potentially cause more problems than it will solve, however it's great that you're keen and looking at ways of getting better - that drive to improve is the first step to you becoming a great climber. For more information on injury related stuff, including prevention, please buy Dave Macleod's new book and read the first couple of chapters on technique and injury prevention (this should be talked about during NICAS level 5 so you'll be getting a head start there!) as well as the chapter on young climbers.
For people your age, the most important thing is to gain a large variety of experience just simply climbing. Teaching your body how to climb, in terms of technique, is what you should focus on currently, this will have the added advantage of preparing and conditioning your body for future training. You should try to understand and apply the fundamentals of good climbing technique, and if possible spend some time with a good climbing coach, who will point you in the correct direction. You're at an age where your body will learn techniques much better than it will in a couple of years time, so use this time now as an opportunity to gain that experience. This is why most of the top climbers now are so good - they started climbing from a young age, so they are naturally smooth and efficient and have a large bank of moves and techniques stored in their head!
The best way to improve your grade in the short term is to climb as much as you can, without burning yourself out! When I was 13-15 ish I climbed 3-5 times a week in my local bouldering wall, every night after school. Now, I always maintain a base level of climbing ability no matter how little climbing I do, because of this period of conditioning and the amount of climbing I did at a fairly young age. Bouldering might be a good way for you to increase the amount of climbing you do and give you a good variety at the same time, so if there's a centre near you (where do you live?) then check it out and have a session of bouldering per week as well as your current climbing sessions. I think you'll see a lot of progress as your technique improves, along with your general upper body strength. Bouldering is also a great way of improving your finger strength too, and can be less intense than fingerboarding alone.
Congratulations on completing level 3 anyway, enjoy level 4 and make the most of the opportunities you will have to improve your climbing, remember though that NICAS isn't the most important thing in your climbing, you need to enjoy it and climb for the enjoyment factor, not just for ticking boxes in a NICAS book! There's a lot of good information out there, look at "The Self Coached Climber" for some really good and detailed technique and training stuff, but be careful as it is aimed at people older than you who aren't still growing. Dave MacLeod's book for injury prevention and advice, as well as the resources offered by NICAS related to training and young climbers - some of the NIBAS material is brilliant too if you can get hold of it.
I hope that some of what I've written will help. Feel free to ask more questions