Is it recommendable using modafinil during exam periods? I'm very stressed and tired

Not indicated. Modafinil although a stimulant and may help with focus etc during exams is not indicated for this purpose. The official indication is for use with narcolepsy, shift work sleep disorder and sleep apnea. I would recommend getting more sleep and staying on a good sleep wake cycle regimen instead. Sleep deprivation is something to avoid and will help you more in the long run.