Le represents relative past tense in this sentence:
明天 你 下了 課 來找我！
míngtiān nǐ xià-le kè lái zhǎo wǒ!
tomorrow 2p.sg leave-LE class come see 1p.sg
‘Come to see me after class tomorrow.’
First, if we omit lái zhǎo wǒ(Come to see me), the sentence would have grammatic mistake.
Second, Le in this sentence indicates that 下(xià)课(kè)(leave class) happened before 来(lái)找(zhǎo)我(wǒ)(Come to see me).
Therefore, although the whole sentence happens in the future tense, 下(xià)课(kè)(leave class) belongs to the past tense comparing with 来(lái)找(zhǎo)我(wǒ)(Come to see me).