Regarding your question, even the Chanuka candles that are lit in a shul should preferably remain lit for a half hour.[1] However, it is probably not responsible to leave the candles unattended in a school building. Therefore, we suggest that you extinguish the lights before leaving, which most poskim allow under such circumstances.[2]
Allow us, however, to raise issues related to your assumption that you should light the candles with berachot. The gemara does not mention lighting Chanuka candles in a shul, but by the Rishonim’s time, it was an accepted minhag. The Beit Yosef[3] cites the Kol Bo that it is intended for those who do not light at home and to increase the level of pirsumei nisa (publicizing the miracle), which is at the heart of the mitzva. The Rivash[4] develops the latter reason beautifully. Originally, everyone lit the candles in front of his house. Because fear of non-Jews forced the lighting inside the home, the minhag developed to ‘spread the light’ at least in shul. Additionally, the Rivash explains that we recite a beracha, as we do for certain other minhagim, and this is the broadly accepted practice.[5] Others explain that since a shul corresponds to the Beit HaMikdash, where the original miracle occurred, it is appropriate to also perform the commemoration there.[6]
Acharonim debate whether it is proper to light Chanuka candles with a beracha at public gatherings not in a shul. Some claim that since the minhag is so novel, we may not extend it further.[7] Others counter that the important matter is publicizing the miracle, wherever the opportunity to do so may occur. If there are people present who have not yet fulfilled their mitzva,[8] or there will be a minyan for Ma’ariv,[9] there are stronger grounds to extend the minhag.
In any case, it appears that the classroom you mention can be considered a shul, certainly if there is a regular minyan there. However, the timing is problematic. The minhag is to light the candles in a shul between Mincha and Ma’ariv,[10] even when this is earlier than one would light at home.[11] This facilitates proper pirsumei nisa until after Ma’ariv, after which people usually disperse quickly.[12] Therefore, it is questionable whether your classroom is like a shul in this regard, since you are not davening Ma’ariv, which is within the time of lighting. One might consider lighting the candles before the d’var Torah, so the children will see them during the subsequent several minutes. If the d’var Torah is to begin before sunset and finish after it, it probably pays to light after sunset.[13] There is also a question whether one needs a minyan of adults[14] in order to light in shul.[15] Depending on the details, there are likely halachic reasons to prefer lighting the candles without a beracha,[16] which also makes blowing them out simpler. However, you may decide that the educational factors tip the scale in favor of doing a regular lighting. For example, one major consideration is whether any of the children come from homes where candles will not be lit
Your e-mail address indicates that you are a rabbi, which should make you particularly suited to decide the halachic elements. In your role as a teacher who “lives” the children’s education, you should also factor in the educational elements of the experience. Whatever you decide about the berachot, you may extinguish the candles for security reasons.