First, don't be discouraged by an "F". Fs happen for a variety of reasons unrelated to talent or hard work. I understand the desire to save paint as a student, but diluting the paint will hinder solid coverage. You may need to apply two coats to get the kind of flat solid you seek. Be sure to let the first layer dry before applying the second. Masking tape may also be useful.

what brand of paint do you use?

Are you sure this is not linguistic confusion? Flat colors may also mean plain color, without nuances or gradients...

Well obviously this looks pretty flat already. The "stroke" inside the flat shape is exactly the characteristic of traditional painting, it gives a unique texture. I don't really see much of any problems here in your photo :D

Hi Miguel, By adding too much water or fluid medium you are unbinding your paint, thus leading to brush marks or a streaky look. If you use too little or no water or fluid medium then the paint won’t spread across your canvas. Instead of mixing your paint with water, try working with a damp paintbrush. Wet your brush in water and then blot off the excess water (you don’t want a wet paintbrush, but a damp one), then go straight into your paint. To get flat colours that are opaque without marks I find that working in layers helps. Further, if you paint upward or downward strokes with your brush and you find streaks, go over it again in a horizontal motion with a soft brush (before the paint dries) to blend out the streaks. Also, pay attention to how much pressure you are applying as too much pressure with your brush can sometimes cause marks in your paint. Finally, if your brushes are very stiff then you risk making marks in your paint. To get crisp flat colours that have a blended look, try working with a brush that has soft bristles. My personal favourite is Heinz Jordan gold sable synthetic brushes. But any brand of synthetic brush that has bristles with good flexibility will work as well. I hope some of these tips help you. Good luck with your school project! 

What sort of brush are you using? What difference are you seeing between water and medium?