Hmm... I don't think it's universally true that all Christians don't get high marks or don't get into their dream college. Some Christians do get in and other Christians don't get in. Also some non-believers get in and some non-believers don't get in.
Everyone is created by God and all their natural talents are given by Him. Some non-Christians use their natural talents and can get into good schools. Christians can use their natural talents too. But Christians can also do better because they can get God's help.
If Christians fail, there is usually another reason. The most common reason is doubt. Even though you say you are afraid to doubt God, I think you are already doubting God a bit when you look at your own situation and compare it to some other people.
Because you are asking this question, your underlying reasoning contains some doubt. You might be thinking God hasn't helped you or won't help you.
However, the Bible tells us:
"But if any of you lacks wisdom, let him ask of God, who gives to all generously and without reproach, and it will be given to him. But he must ask in faith without any doubting, for the one who doubts is like the surf of the sea, driven and tossed by the wind. For that man ought not to expect that he will receive anything from the Lord, being a double-minded man, unstable in all his ways." (James 1:5-8)
So if you doubt, your prayer is not effective.
The Bible says:
"And without faith it is impossible to please Him, for he who comes to God must believe that He is and that He is a rewarder of those who seek Him." (Hebrews 11:6)
So when you come to God in prayer, you must believe He is God and He will help you.
Faith is not a feeling. Faith is also not something you muster up. Faith is a choice. It is choosing to believe God's words and not believe in any words or thoughts that contradict His words.
Faith grows with practice. You can practice trusting God for everything. Ask for His help for things big or small.
Ask Him everyday. See every challenge as an opportunity to ask for His help. See every negative emotion as an opportunity for Him to step in and give you peace. The more you ask, the more you will receive.
Ask without giving up.
You can even ask God for more faith and wisdom!
When you encounter a problem (like trying to get high marks on a test), don't see the problem as a huge thing. Don't focus on the problem. Focus on God and how big He is. When you see a big God and a tiny problem, you are practicing faith. But when you see a big problem and a small God, you are doubting.
So focus on our big God and don't focus on your problem. And God will reward you.
There is plenty of opportunity to practice faith. Start doing that now. It doesn't matter what marks you got or where you are now. God is able to change everything. So practice these things and start believing.
"Trust in the Lord with all your heart
And do not lean on your own understanding.
In all your ways acknowledge Him,
And He will make your paths straight." (Proverbs 3:5-6)