There can be any number of classroom based research topics that you can work on. The most important point is what interests you or what capabilities/resources you have. That can narrow down the list of potential topics. For example, you may be interested in studying the behavioral dynamic in your classroom, however your current class is a group of the most angelic students that you can ever encounter. In that case, you simply cannot work on behavioral dynamics of your class, unless you have data from elsewhere.
Apart from your interests and resource availability, think of specific topics that can be completed within a given amount of time (say a semester or an academic year, etc.). For example, instead of choosing a vast topic like "online education", select something specific like "do online education results in higher score in standardized tests (such as SAT)", etc. Similarly, a topic like "do open-book exams lead to better learning?" is tractable within a semester or so.
Another part of selecting a topic is checking if you have the adequate resources (and permission, if any required, say from parents/supervisor) and making a timeline for completion of key milestones.
Talking to colleagues or friends can also help you narrow down the topic.
Here are some topics that you may work on (again, the list is just informative, your interests will govern what you want to work on):
- What is a better assessment method: online tests or off-line tests?
- How can technology be incorporated in the classroom?
- Do field trips really add to student's knowledge?
- Do students learn more from each other than the teacher?
- Are charter schools better than public school?
- The impact of social media and internet on the way the students interact with each other.
Hope this helps.