Social interaction with the local communities plays a fundamental part in ensuring that you all get the most out of the experience. To learn about the local traditions and culture is an important part of volunteering abroad and can do nothing but promote good relations between all concerned. Volunteering is not only about your experiences, but also about what you can learn and equally what you can leave behind.
Working in partnership with a local organization is a great way to make sure that the locals have a vested interest in the project. You need to be sure that the local community believes in what you are doing. Do they support your work? If the local community is not involved in decision-making or the work itself, then what makes you think they actually want the well/school/playground you are building?
Is the placement available year round? If not, is there a valid reason for this? What happens for the rest of the year? Some projects have no need to be continuous placements, but there should always be continued support otherwise there is unlikely to be any sustainable change to the community and distrust will have been created been the community and organization. This is why asking about the short, medium and long term goals of the project is absolutely vital to understanding the level of integrity held by the organization.
If you are looking to teach for less than 1 school year this is probably not a good idea. Continuity is extremely important for learning. If you just arrive at a school and teach for 4 weeks, how will the students react to your teaching style? Will they have time to get used to it? More importantly, once they are, they will then have to get used to someone else with another teaching style after you leave.