Sooner or later, life gets more uphill than before. High school is there to prepare you for real life – dealing with employers, business associates, customers, fellow workers, new acquaintances, and so on.
The curriculum in high school is more difficult, that's true. However, teachers are aware of this, and if you make an honest effort to get up to speed, they'll see that.
As for friends, seeing them isn't so automatic as it was. You'll need to plan to get together, whether simply walking to school and back, or seeing them for shopping, hanging out at each others' homes, or going places together. This is a life skill to learn, too.
If you did reasonably well in elementary school, you'll be okay in high school as soon as you adapt to the new system, buckle down, and keep up with your studies. You can do this – almost everyone else your age has managed quite well.