I know someone who teaches a weekly Kiruv and Hebrew class and wants to light a menorah before the class, to show them how. However, this class only lasts about and hour long, so is it permissable to cut Chanukah candles so they won't burn for quite as long?
(If the Chanuka menorah is being lit in the daytime, it should not be lit with a bracha.) (Shulchan Aruch 672/1)
If it will be lit in the evening, from a half hour before tzeit hakochavim (night fall) until after nightfall, it must be lit for 1/2 hour. (Shulchan Aruch siman 672/se'if 2.) After 1/2 hr, it may be extinguished, (ibid, see also Mishneh Berura, sub-seif 7, the one lighting should have in mind when he lights that he will extinguish it after 1/2 hour.)
As for the kiruv factor, it would be necessary to announce at the lighting that "we are lighting for the minimum of half hour" so that people understand. (See Mishnah Berura, 672/sub-se'if 8, based on Magen Avraham and Eliya Raba, quoting the Maharshal.)
If the candles are cut and they cannot last for the entire half hour, then the lighting is improper and berachot should not be said. (Mishneh Berura 675/sub-se'if 8, based on the Chemed Moshe; however the Pri Chadash allows one to add oil after the blessing is made.) In addition, it should be kept in mind that the more candles, the more heat. So even if the candles are cut to the length that one candle would stay lit for 1/2 hour, three or more candles burning in proximity will probably not last for the entire 30 minutes.
At least one Chanuka candle (not including the shamash) must be long enough to last for the entire 30 minutes, under the conditions.
According to the Rambam (brought in the first Biur Halacha on Siman 672), the time for lighting is a bit more than 1/2 hour, and must specifically be lit from the time that the sun goes below the horizon until 30+ minutes afterward, which he identifies as "the time that feet are finished from the street/market." (This to allow pirsumei Nissa (publicizing the miracle to those who are still walking outside.) If one misses that very short time period, he would not be permitted to light. To my knowledge, that is not the Halacha, even among the Sefardim, who normally follow the Rambam. Rav Yosef Karo clearly states that these times are "lechatchila" not "b'dieved." He adds, "if someone is busy/overwhelmed and is concerned that he will not be able to light once sunset arrives, he may light up to 1 1/4 "relative hours" (here, c 52min) before sunset! There might be a case to apply this rule is the class is taking place only from say 45 minutes before sunset until shortly after sunset. (Mishneh Berura. sub-se'if 2). The Mishneh Berura (672/3) allows one to make the berachot in that situation.
The Rema, based on the Tur, who quotes Baalei Tosafot, states (gloss on 672/2) that we (at least B'nei Ashkenaz) are not concerned with the standard of "the time the feet are finished,etc," since by the time of the Rema, et al, the custom was that we lit inside the homes, not at the doorway. Therefore the pirsumei Nissa is done for the household not for those in the marketplace.
Certainly we should prefer to light at the correct time, and one should do next to nothing (including learn Torah, eat, etc MB 672/ss10) before lighting (so as not to get caught up and miss the time) but if "things happen," and one did not reach home until late, he may light all night (SA 672/2) until [half hour before] dawn (MB 672/ss 11) with berachot (Pri Chadash, Magen Avraham) preferably with the household awake. If they are "unwakeable," the Chemed Moshe holds that one should light anyway, with a beracha, since a single person (who has no guests) would light alone anyway. (Sha'ar Tziyon, 17)