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The magnetic moments of ^N and ^B were measured by using spin-polarized radioactive nuclear beams which were obtained from the projectile fragmentation reaction. The observed magnetic moment of ^N, |μ(^N)|=(0.352±0.002)μN, where μN is the nuclear magneton, falls outside the Schmidt lines. By virtue of a simplifying feature of nuclear structure inherent in a p_ valence nucleus, the deviation from the Schmidt value is attributed on firm ground to admixing of the configurations in which two neutrons in the sd shell are coupled to J^π=2^+. This interpretation is confirmed in standard shell-model calculations. The calculations reproduce fairly well the experimentally inferred amount of 2^+ admixture, as well as the experimental magnetic moment itself. The magnetic moment for ^B was determined as ‖μ(^B)‖=(2.545±0.020)μ_N. The result is substantially smaller than the πp_ single-particle value, and the shell-model calculations indicate that the quenching of μ largely stems from J^π=2^+ configurations of the sd neutrons. The observed amount of quenching, however, is larger than the shell-model predictions, suggesting an enhanced contribution of the 2^+ neutron configurations. This result is explained if the pairing energy for neutrons in the sd shell of a neutron-rich nucleus is assumed to diminish by about 30%. We also find that the use of the reduced pairing energy improves agreements in the magnetic moment and low-lying energy levels of ^N as well.