Optical Clocks and Stars
I'm not entirely sure about the technological requirements, but I'm pretty sure that the Greeks had access to optics, (At least they get it pretty quickly when I play Civ). That said, I think that having a way to use a magnifying glass in a similar method to a sundial might work. Probably cheaper than all the candles, which was my first thought. But the magnifying glass would be reusable, additionally, because you could concentrate the light into a really small focal point you would be able to have much smaller marks on your "sundial." So for example, every inch could have a mark that is a half an Earth day. And you could have smaller marks for more precise units, like quarter days, hours, etc.
I would hope that the path of the sun was more or less consistent, but even if it wasn't it could probably be accounted for. At night, they would likely use the stars positions, as denoted in Cyrus' answer. Issues would of course be things like overcast and inclement whether. Also, I'm not really up and up on the physics of optics, so I'm not entirely sure if you would need a really large magnifying glass to capture and focus the light into a small enough point for it to be useful, and it could get quite hot, so you might need to make sure there's a nonflammable stone or metal to use for the marks. This would also need to be outside in a relatively safe spot where it can be guaranteed not to be tampered with (glass and all that).
If all that seems workable, I think Magnifying and star positions could be a good choice.
As a second choice:
Foucault Pendulum Clocks
Perhaps your people have the technology necessary to create one of those. Since you have a day night cycle, you are likely to have a rotating planet. This can be used both day and night, but it would require some way to replace the energy lost each swing without disturbing the path which is tricky given limited technology. Also, this would likely need to be HUGE to have enough meaningful markers on it so you might just have one in the center of the village or city. Additionally, you will almost certainly have to hire guards to make certain no one messes with it (because you just know some delinquent trying to skip class or work is going to try and speed up time). This one is certainly more of a stretch, but if they could make it work, the clocks could be become something of an interesting city center for tourists.
But yeah, give those a shot and see how they work for you.