Heritage language situations are defined as sociolinguistic contexts where a given language is asymmetrically situated with respect to another (or other) language(s) (Rothman et al 2023: 321). Accordingly, heritage bilinguals speak a language at home which is different from the majority language within the region, and hence can be considered minority speakers. Here, we explore the mental lexicon of three groups of heritage learners, Arab (n.69), Rumanian (n. 21) and Urdu (n.8), within the majority Spanish school context. All participants attended 4^th grade of Secondary Education (equivalent to grade 10) and learned English as a foreign language as a school subject. Both the lexicon in the majority language, Spanish, and the foreign language, English, are examined via a semantic fluency task. Our analyses are framed within graph theory and complex network systems, and we provide graph metrics and graph figures to substantiate our claims. Results reveal lack of differences in quantitative and qualitative terms within the heritage learners and when compared to Spanish majority speakers (n. 100). Our findings are in line with previous research results which failed to find significant differences between heritage speakers and their majority peers in the acquisition of an additional language (cf. Siemund and Lorenz 2024, Rothman et al 2023). Situations of subtractive bilingualism, the impact of a very early schooling in Spain, and other socioeconomic and sociolinguistic reasons (neglect of heritage languages, attitudinal and affective aspects) are brandished as possible explanations for the results. This study has high ecological and educational validity since it presents a picture of the lexical knowledge and semantic structure of the EFL learners in real secondary school classrooms reflecting thus the multilingual ecosystem of the region.