My Friends Always Leave Me Out! Any Advice?

I’m in year 10 and in a group of friends about 8-10 and I feel like I’m constantly being left out. We’ll go on school trips and I’ll usually be the one left out because nobody offers to pair up with me and when I offer they say they might be with someone else which they obviously may not be. Then they’ll make plans in the weekend and don’t bother inviting me so I’ll end up hearing about this through Twitter and the only time I get invited is on birthdays or if I make a plan and invite everyone. I’m just sick of it and it’s been two months and I haven’t received a gift from two of my “friends” for my birthday which was two months ago even though I gave them all thoughtful presents. I just don’t what to do. A few months ago I told them I felt left out and they began inviting me places but now the whole abandonment thing has started again and it’s frustrating. The only time I’m noticed is if they’re stuck on school work because I’m the smartest – I’m not being big headed or anything. In the summer not one person contacted me at all and when we returned to school it was like nothing happened at all. None of them bother contacting me during school breaks. Why should I make the effort all the time? And when I do go off and ignore them for a bit they say I’m moody but I’m not, I’m clearly upset. What shall I do?