。
I think she was conscious of being stared at by all those boys.
Literal
She [topic-は] many [genitive-の] boys [by-に] is-being-stared-at [nominalizer-の][object-を] was-conscious-of [quotative-と] think.
Multiple layers of subordination, all worth tracking. The innermost clause 大勢の少年たちに凝視されている is the passive 凝視される ('be stared at') in ongoing-state ~ている form, with the agent ('many boys') marked by に. That whole clause is nominalized with の and made the object of 意識する ('be conscious of'). 意識していた is the past progressive of 意識する. Finally, the speaker reports the whole thing tentatively with と思う ('I think'). One sentence, five grammar layers — typical of literary Japanese.