Huelva is a small and convenient city, and from most parts of town, it is an easy walk to arrive at Estuaria. If you have a bike, even better! From the center, it is also easy to come on the bus, though walking may often be about the same duration or faster.
The city buses have stops for the 4 line, the 8 line, and the 2 line that are near to the school. From other parts of the city, the 1 or the 3 would get you close to the school. The local bus company, called EMTUSA, has an app which provides accurate bus schedules and the current location of all buses for all lines. Google Maps is not reliable for city bus schedules around Huelva, though it works for directing you to the closest stop and most convenient routes by bus.
The bus costs 1.10€ paid only in cash, or 0.65€ using the city bus card, which can be purchased for only a few euros in some tabacco shops, called 'estancos', and refilled in person there or online.