It sounds like this project is very interesting! To answer your first question, in the "background" section of the project, there is a link about "How Many Survey Participants Do I Need?" : https://www.sciencebuddies.org/science- ... ze-surveys
Although there is no single correct number, the amount of people in a classroom depends on the margin of error for which you are aiming. In general, for a smaller amount of error, you want a larger sample size. However, you may be limited by the amount of students that are in the class.
For your second question, you should ask the teacher for permission to conduct the experiment beforehand. That way, the teacher will know not to have any important lesson plans during that time period. If the teacher does not want his or her class interrupted, you can simply find another teacher who is willing.
Thirdly, you should not tell the participants that you are giving them a survey. As the procedure states, you should have the teacher give sealed envelopes to the participants with instructions on when to open them; the participants should not know they are taking a survey until they open the envelopes, as you are testing how well they remember an arbitrary incident, not how well they can recall an incident about which they know they will have to answer questions later.
As for your fourth question, the answer is similar to the previous question: no, you should not inform the participants that a random person is going to enter. As previously stated, you are testing the accuracy of eyewitnesses' memories, people who do not know an event such as a crime is going to happen until they actually see it. In some cases, eyewitnesses may not even realize the importance of what they are seeing, which makes it all the more important that you do not tell the participants to memorize features for a survey later.
Finally, your survey questions should be mostly open-ended, as you want to ascertain what the participants actually remember. If they are given multiple choice questions, they may guess correctly out of pure luck and you will not be certain how much they recall on their own. If you are uncertain as to what types of questions you should ask, the procedure section of the project gives some examples, such as "What time did the person enter the room?" and "What did the person say to the teacher?": https://www.sciencebuddies.org/science- ... #procedure
The section also explains some ways to analyze the accuracy of the data you obtain. For further suggestions about formulating survey questions, this Science Buddies link may help: https://www.sciencebuddies.org/science- ... n-a-survey
Also, remember that when you are performing projects with human subjects, you will need to obtain permission from the children's parents and teacher. The procedure section of the project has an informative box at the very top explaining the requirements. Additionally, you may want to peruse this link to the requirements for human-related projects, especially if you are considering entering your project in a competition later on: https://www.sciencebuddies.org/science- ... egulations
I hope this helps! Have fun completing your project!