1. The teacher was bad/unfair.
That excuse distracts from what is in your control to worry about what is out of your control. If you give a consistent effort and come to class with a confident, humble attitude, even an unfair teacher will have a difficult time treating you poorly. Focus on your own attitude and effort.
2. I didn't have enough time.
This means that you haven't fit school into your priorities. Instead of using time as an excuse, use it as an opportunity to improve your time management. Write out your daily schedule and fit homework & studying into it on a regular basis. Schedule more time than you think you will need.
3. I'm not smart enough.
Not being smart enough should actually become motivation, not an excuse. If you feel like a class is moving too quickly and you are having trouble keeping up, it's time to study more at home. You could also look for peer tutors and extra help from teachers to stay on top of your game. Look for the challenge!
4. The class is boring.
Are your goals boring? You won't be completely interested in every class or lecture. The way to beat boredom is to be so motivated in reaching your goals that you are willing to push past boredom and stay engaged. Come in with positive body language and find ways to refocus when you are distracted.
5. When will I need to know this?
Sometimes, you will learn important information that will be relevant for your whole life. Other times, you are learning the process of learning and getting better at gaining and retaining new knowledge. Even if a topic doesn't seem important to you, engaging in the learning process will make you better at learning, which will pay huge dividends when you are dealing with a subject of importance!