I'm new using ArcMap and I have a GDB of schools (and its enrollment data) at block level. What I need to do is to find the places with the most concentration of enrollment in a radius of 5km. Since I'm new using this tool I'm completely lost.
I would suggest reading up on spatial statistics. In particular, I would look at spatial autocorrelation and clusters. The ArcGIS Resource page on the Spatial Statistics toolbox might be a good place to start.
As you suggested in a comment to the question, convert each enrollment value in the GDB to an enrollment density by dividing it by the block's area. Convert this layer to a grid. Compute its focal sum using a 5 km radius. Multiply that, if you like, by the area of each grid cell (equal to the square of the cellsize). Notice that this cancels out the preliminary division-by-area: you have now produced a grid whose cell values are total enrollment within a 5 km radius. Highlight the cells with the largest values: these are your "hot spots."
You will need access to a raster-based GIS, such as Spatial Analyst, to carry this out. Open-source solutions exist in GRASS,
R, Python, and other platforms.