Observations once a week should be more than sufficient. You may wish to repeat this experiment once during the cool months and again during the warmer months, preferably for a total of four times a year so that you can compare their performance in each season. You may wish to change the crops. The four chosen are very easy to grow. Each has different temperature requirements to reach harvest.
Corn needs the hottest weather.
Cilantro and radishes will grow in cooler weather. Tomatoes are difficult to grow from seed. This is a good opportunity to discuss New World food contributions. Set up the stick for altitude angle permanently in a location not impeded by shade, preferably in the garden itself. Teach students how to calculate the angle in the classroom.