The mishna at the start of chapter 13 of Y'vamos cites a disagreement between the schools of Shamay and Hilel about a minor girl who is married off by (for example) her mother and can refuse the marriage, thereby annulling it. The commentators on the mishna, following Ula in the Bavli, explain the dispute as follows: The school of Hilel maintains that if the girl refuses the marriage, then she can be married again and can refuse the second marriage also, and so on. The school of Shamay retorted "the daughters of Israel are not wanton!" and should not have successive annulled marriages. Rather, after her first refusal her next marriage must be irrevocable, and they describe two ways it can be made so: she can wait to marry until she's no longer a minor, or she can marry completely (rather than merely be "betrothed"): either way, they say, is irrevocable.
Because the school of Shamay describes these two ways of ensuring a second marriage is irrevocable, it sounds to me like they would agree that, were these steps not taken, the second marriage would indeed be revocable. In other words, if she were betrothed again as a minor and then refused the betrothal, even the house of Shamay would hold the refusal to be effective. Is that so?