NEW YORK ( TheStreet) -- Summer is in full swing, and many offices nationwide offer "summer Fridays" -- flexible Friday schedules that allow employees to work a half-day or take the day off in exchange for additional hours worked during the week. If you work for a company that offers flexible hours, you're in luck. But if you're stuck at work until 5 p.m. every Friday through September, chances are you may be green with envy -- or taking matters into your own hands, which experts say can be a risky proposition. Approximately 30% of employees report leaving early on Thursdays and Fridays during the summer whether or not their manager approves such activity, according to workplace technology company Captivate Network. A further 14% of employees say they take unneeded sick days on Mondays and Fridays during the summer to gain a "free" three-day weekend. "Going into the summer, employees should know whether or not they have summer Fridays or the ability to leave work early if all of their tasks are done," says Michael Crom, executive vice president at Dale Carnegie Training. No matter how flexible your jobs have been in the past, employees always need to ask their supervisor for permission to leave early. Sneaking out early when the rest of the office is still on duty can be a firing offense, and calling in sick when there's not a need can also be grounds for termination. "Unfortunately, if an individual works at a company that maintains regular work hours and a friend of theirs works at a different company that offers summer Fridays, there is not much they can do," Crom says. "Employees should always feel as though they can ask for a half-day if they need to catch a ride or if they have a strict departure time such as a flight or train to make, but cutting out early should never happen unless a boss grants permission." Employees should always treat Summer Fridays as a perk -- not a right, Crom says. If you're finished with your daily workload, you can feel free to leave the office, while still remaining available if a client or colleague needs you.