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If the tuition and other college expenses paid by the trust using corpus - that is not his taxable income.
Also if the trust has taxable income - generally the trustee has a choice - either the trust pays income taxes on that income or the taxable income is passed to beneficiaries or a combination of both.
The beneficiary is only responsible to include into his/her income taxable part of the distribution reported on K-1.
All other distributions are not taxable for beneficiaries.