What is the Attack Range of a Zombie in Minecraft?

Given Minecraft Bedrock Edition and Minecraft Education Edition are built on the same platform then the addon documentation for Bedrock Edition can be applied to Education Edition.

The vanilla mob examples for the Zombie in the documentation here does not contain a "reach_multiplier" key. This key is how far out of a mobs hitbox they can hit. The default value is 2 Therefore, Zombies can hit players who are twice their hitbox away.

Taking this further and looking at the Zombies remaining behaviour data here the width collision box is "0.6" meaning it is a square with 0.6 block sides, given that this box is centred around the zombie and the player has the same hitbox size a zombie can hit you from 0.6 * 2 = 1.2 blocks away. That is when a Zombie approaches you head-on such that the two square hitboxes are at the same angle.

Some trigonometry can determine a Zombie may be able to increase this distance if it approaches from a corner: sqrt( 0.6^2 + 0.6^2 ) * 2 ≈ 1.697 blocks. This is therefore the maximum distance you could be hit, with 0.6 the minimum.

You could use a behaviour pack to edit the behavior.melee_attack of the zombie and increase the reach_multiplier value.