This all depends on the type of school and current ownership of the land.
For community schools the land is generally owned by the local authority. Upon conversion, the Academy trust will occupy the land on a 125 year lease with the local authority becoming the landlord for the Academy Trust.
Land ownership for Church schools is often split between the diocese and the local authority. For land owned by the local authority the 125 lease ruling will apply. If the diocese owns the land a license will generally be granted to allow the Academy trust to use the land under the church supplemental agreement.
For Trust schools the freehold of the land will be transferred from the current foundation to the Academy Trust resulting in the new Academy Trust becoming the owner of the land.