Yevamos, Chapter 13, Mishnah 1. This chapter deals with the laws of a ketanah - a girl who has not yet reached the age of mitzvos and was married off not by her father but by her mother or her brothers, who have the power to marry her off only by rabbinic law. As long as she is a minor, she can leave this marriage without a get, by means of 'mi'un' - a refusal that annuls the marriage. Our mishnah opens with the laws of mi'un: in what manner and under what conditions may the minor refuse and annul a marriage that is only rabbinic.
The first dispute - at what stage mi'un may be performed:
"Beis Shammai omrim: ein me'manin ela arusos" - according to Beis Shammai, mi'un is possible only at the stage of betrothal, when the minor was betrothed with rabbinic kiddushin, but not after she entered the chuppah and began living with her husband. Once the marriage has taken place, there is no longer room for mi'un and she requires a get. "Beis Hillel omrim: arusos u'nesuos" - in their view the law of mi'un applies even after the marriage has taken place.
The Gemara explains the reasoning behind the dispute: Beis Shammai were concerned that a man would not want to enter into the expenses of arranging a wedding and a feast, knowing that his wife might refuse and lose everything in a single moment. This concern is liable to prevent people from marrying her, for who would want to lay out a great deal of money on a marriage that could be undone by a simple refusal. And since the Sages instituted marriage for her by rabbinic law for her own benefit and protection, they did not want that very enactment to work against her and make it harder for her to be married - therefore they ruled that mi'un is possible only at the stage of betrothal, before any money has been spent. Beis Hillel, in contrast, were not concerned about this and did not see it as an obstacle to her being married.
Three further disputes in the laws of mi'un:
"Ba'ba'al v'lo ba'yavam" / "Ba'ba'al u'va'yavam" - according to Beis Shammai, mi'un is performed toward the husband alone, and not toward the yavam. If her husband died while she was still a minor, she cannot refuse the yavam; and since she is a minor she also cannot perform chalitzah, so if she does not wish to enter into yibum with him she must wait until she reaches the age of mitzvos, and only then will chalitzah be performed. Beis Hillel disagree and hold that mi'un is performed toward the yavam as well, and in this way she is released from the bond.
"Befanav" / "Befanav v'shelo befanav" - according to Beis Shammai, mi'un is valid only in the presence of the husband, and this is deliberate: when she has to refuse in his presence, it is more embarrassing for her and will hold her back from refusing. Beis Hillel hold that even not in his presence her refusal is a valid refusal.
"Beveis din" / "Beveis din v'shelo beveis din" - according to Beis Shammai, mi'un must be done before a court of experts, whose ordination traces back to Moshe Rabbeinu. Beis Hillel hold that mi'un is valid whether before a court or not before a court. And the practical halachah is that it is enough for the mi'un to be done before two people.
"Amru lahen Beis Hillel l'Veis Shammai: me'manes v'hi ketanah, afilu arba v'chamishah pe'amim" - according to Beis Hillel, the minor can refuse again and again, even four or five times: she refuses one husband, marries another with kiddushin and nisu'in, refuses him as well, and so on with many husbands.
"Amru lahen Beis Shammai: ein benos Yisrael hefker" - hefker means something that has no owner, something no one wants. The daughters of Israel are not hefker, and a woman must not be permitted - even though she is young - to move from husband to husband without a get.
Therefore, even according to their view that mi'un is possible only from betrothal, it may not be done more than once. Rather, say Beis Shammai, if she wishes to leave her first marriage - she refuses and waits until she grows up, and once she matures she can no longer refuse in any case, and she does not marry again until she is grown and ready for it. Or alternatively, she refuses and marries - she refuses now and marries another with a full marriage, after which, according to Beis Shammai, there is no possibility of mi'un at all.
In summary: in this mishnah we learned the four disputes of Beis Shammai and Beis Hillel in the laws of a minor's mi'un: at what stage she may refuse (only while betrothed or even once married), toward whom (the husband alone or the yavam as well), where (in his presence or even not in his presence, before a court or even not before a court), and how many times - with Beis Hillel permitting repeated refusals, and Beis Shammai responding "ein benos Yisrael hefker", and therefore she refuses only once: either she waits until she grows up, or she marries with a marriage after which there is no mi'un.