We consider a deep ReLU / Leaky ReLU student network trained from the output of a fixed teacher network of the same depth, with Stochastic Gradient Descent (SGD). The student network is over-realized: at each layer l, the number n_l of student nodes is more than that (m_l) of teacher. Under mild conditions on dataset and teacher network, we prove that when the gradient is small at every data sample, each teacher node is specialized by at least one student node at the lowest layer. For two-layer network, such specialization can be achieved by training on any dataset of polynomial size ( K^5/2 d^3 ϵ^-1) (sample size including augmentation) until the gradient magnitude drops to (ϵ/K^3/2√(d)), where d is the input dimension, K = m_1 + n_1 is the total number of neurons in the lowest layer of teacher and student. To our best knowledge, we are the first to give polynomial sample complexity for student specialization of training two-layer (Leaky) ReLU networks with finite depth and width in teacher-student setting, and finite complexity for the lowest layer specialization in multi-layer case, without parametric assumption of the input (like Gaussian). Our theory suggests that teacher nodes with large fan-out weights get specialized first when the gradient is still large, while others are specialized with small gradient, which suggests inductive bias in training. This shapes the stage of training as empirically observed in multiple previous works. Experiments on synthetic and CIFAR10 verify our findings.