A schema is a blueprint of the database which specifies what fields will be present and what would be their types. For example an
employee table will have an
employee_ID column represented by a string of 10 digits and an
employee_Name column with a string of 45 characters.
Data model is a high level design which decides what can be present in the schema. It provides a database user with a conceptual framework in which we specify the database requirements of the database user and the structure of the database to fulfill these requirements.
A data model can, for example, be a relational model where the data will be organised in tables whereas the schema for this model would be the set of attributes and their corresponding domains.
References: Understanding the schema and Database System Concepts (H Korth and A Silberschatz)