This study investigates the effects of weekly heritage language (HL) classes on first language (L1) proficiency in speakers who arrived in the second language (L2)-dominant setting before the onset of puberty. Two groups of L1 Spanish--L2 Swedish bilingual high school students living in Sweden participated in the study. One group currently attended HL classes once a week, whereas the other group was no longer doing so. The two groups did not differ with regard to the total number of years of HL class attendance, age of arrival in Sweden, length of residence or degree of L1 contact. Results from a grammaticality judgement test and a cloze test showed that the group that currently attended HL classes outperformed the non-attending group. Using a framework that emphasises heightened attrition susceptibility among speakers who lost contact with the L1-dominant setting before puberty, the study suggests that HL classes function as a factor that, all other things being equal, may counterweigh attrition susceptibility. Moreover, it is suggested that the effects of HL classes on L1 proficiency are short term rather than long term. That is to say, once attendance ceases the counterweighing effect is less visible. (Contains 1 figure, 6 notes and 3 tables.)