Kosher is not food "blessed by a rabbi." In the most general terms, it is something that is certified by a rabbi as being in accordance with the law as set forth in the Torah and in Jewish tradition (which may mean slaughtered, or cooked, or grown, or of the substance, or made, etc.). "Blessing" has nothing to do with it. Moreover, just as pork is prohibited under Jewish law, it is also forbidden under halal, which is derived from the Torah (i.e., the Jewish Bible, the Pentateuch, or Old Testament to Christians). Thus, many old traditional English "delights" :-) such as bangers and mash, would not be permitted to be served in an English school that imposed a halal menu on its students.
The bangers could be made with beef or veal, but if the traditional recipe involves using blood, as so many traditional English "delicacies" do, then it would be neither kosher nor halal.
After googling around a bit, I've found that there are (no big surprise) varying degrees of how strictly halal restrictions are observed, and which fatwas are observed. Most Muslims believe that animals have to be slaughtered by "people of the book" -- which includes Jews and Christians -- and that the name of Allah must be spoken over the meat, but that can be done at any point before eating, not necessarily during the slaughter. Under the stricter interpretations of both kashrut and halal, prayers must be spoken before and during the slaughter.
Halal certified products are only now becoming fairly common in some parts of Western countries, so practicing Muslims have traditionally bought kosher. As do many goyim, because of the perception that the quality is better; at the very least, kosher products are subjected to an additional layer of inspections and monitoring above and beyond the USDA and local health officials.
The biggest difference I've found is that, unlike kashrut, halal has no restrictions on dairy or on leavening agents (the latter of which which only applies to Jews during passover). Oh, and it bans outright any intoxicants, including alcohol, but that shouldn't be an issue in school lunches. Even in Europe.
With halal certification, as with kosher, private groups do the certifying and give permission to use their seal. They have clergy who may or may not lead congregations; many kosher certifying rabbis have it as a full-time job. With small butcher shops and the like, I assume they just get a local imam or rabbi to sign off.
Wikipedia has an article specifically comparing and contrasting halal and kashrut laws. Wikipedia grain of salt implied, of course.